gameplay (gameplay.SC4)
Your objective: To have at least 1,250 guests in your park at the end of October, Year 3, with a park rating of at least 600.
All scenarios are stored in the "Scenarios" directory, and are dynamically indexed in the "sc.idx" file. Only five "unfinished" scenarios of each category will be available at any one time. Most scenarios can continue to be played past the end point, even if you fail to meet the scenarios goal (i.e. win). You do not have to play scenarios in any particular order, and you can delay playing scenarios indefinately. The "score" of a "won" scenario is kept however, so if you play the scenario again, you must beat the previous score in order to "win" the scenario again (winning consists of hand-clapping by all your guests, releasing of any balloons carried by your guests, and a recording of your score).
Some scenarios may include "handicaps". The hadicaps are: Land Modification prohibited; Tree Removal prohibited; High Construction prohibited; and Advertising prohibited.
There are twenty-one Original Scenarios, stored in the files "sc0.sc4" - "sc20.sc4".
There are thirty Corkscrew Follies Scenarios, stored in the files "SC40.SC4" - "SC69.SC4".
There are thirty Loopy Landscapes Scenarios, stored in the files "SC22.SC4" - "SC31.SC4" and "SC80.SC4" - "SC99.SC4".
There are three official "Real" Park Scenarios, stored in the files "Alton1.SC4", "BPB1.SC4", and "HEIDE.SC4".
There are two official Extra Scenarios. One is stored in the file "Fort1.SC4". The other (Mega Park) is stored in the file "SC21.SC4", which is created by the game upon compleating all 21 original scenarios.
gameplay (gameplay.SC4)
Your objective: To have at least 1,250 guests in your park at the end of October, Year 3, with a park rating of at least 600.
PC Gaming World (Pcgw.SC4)
Your objective: To have at least 1,250 guests in your park at the end of October, Year 3, with a park rating of at least 600.
PC Player (pcplayer.SC4)
Your objective: To have at least 1,250 guests in your park at the end of October, Year 3, with a park rating of at least 600.
Steel Twister Roller Coaster Competition (Comp-st.SC4) [slot #110]
Your objective: Build the best Steel Twister Roller Coaster you can!
Go Karts Competition (sc114.SC4) [slot #114]
Your objective: Build the best Go Karts you can!
Steel Corkscrew Roller Coaster Competition (sc116.SC4) [slot #116]
Your objective: Build the best Steel Corkscrew Roller Coaster you can!
Mine Train Roller Coaster Competition (sc117.SC4) [slot #117]
Your objective: Build the best Mine Train Roller Coaster you can!
Steel Roller Coaster Competition (sc119.SC4) [slot #119]
Your objective: Build the best Steel Roller Coaster you can!
Bobsled Roller Coaster Competition (sc120.SC4) [slot #120]
Your objective: Build the best Bobsled Roller Coaster you can!
Inverted Roller Coaster Competition (sc123.SC4) [slot #123]
Your objective: Build the best Inverted Roller Coaster you can!
Suspended Roller Coaster Competition (sc124.SC4) [slot #124]
Your objective: Build the best Suspended Roller Coaster you can!
Stand-Up Steel Roller Coaster Competition (sc125.SC4) [slot #125]
Your objective: Build the best Stand-Up Steel Roller Coaster you can!
Wooden Roller Coaster Competition (sc126.SC4) [slot #126]
Your objective: Build the best Wooden Roller Coaster you can!
Steel Mini-Roller Coaster Competition (sc127.SC4) [slot #127]
Your objective: Build the best Steel Mini Roller Coaster you can!
Prior to the release of the "RCT Save Game Modifier" utility, programmer Henry Winklestein released an unofficial sandbox "Competitions" scenario.
RCTOA Acres (rctoa.SC4) [slot #100]
Your objective: Have Fun!
This "RCT-acCeSS" utility lets you permanently 'crack' the RCT Scenarios so that ALL of them are available at ANY time. An 'undo' function also lets you reset scenario progress to the way that the game's developer intended. Additionally, if you need a 'sandbox' to practice your RCT skills, the program can crack open JUST the RCT Mega Park.
Alternatively, you can download to your "Saved Games" directory these 'almost finished' saved games gathered from the internet ("SavesOriginal.zip, "SavesCF.zip, "SavesLL.zip "SavesReal.zip, and "SavesExtra.zip), that you can use to WIN (and therefore make always available) any/all of the games official scenarios. Many are worth examining for good ideas to use in your own parks. Note that the Mega Park will be created for you by the game only after first compleating all of the Original scenarios.
Most of these Saved Games are only a few moments from compleation, and all you have to do is wait. A few require you to open one of the CLOSED rides in order for the scenario to complete. Once you WIN a scenario, the saved game can be deleted. Note that if you choose to again play any WON scenario, your own WIN will only be officially recorded if you exceed the score of the previously WON scenario.
There is no mechanism within the game to modify a Saved Game or create a Scenario. For a long time the only way for a player to create a "pseudo-scenario" was to take a standard scenario (Razor Rocks was poplular), change the landscaping by spending money, then release the final "workbench" as a "Saved Game" (SV4 file).
Just prior to the release of RCT2, the "RCT Save Game Modifier" utility was created. This utility allows you to create new SV4 files with random terrain. It is also allows you to save a file as a true SC4 Scenario file (they will appear under a new tab called "Competitions"). Custom scenarios created this way must use one of the slot numbers originally reserved for the official Hasbro Interactive competition scenarios (i.e., scenario slot numbers 100-127).
Tip: When creating a custom scenario, it is good practice to include in the scenario name the scenarios slot number (i.e.: "sc123.sc4", etceteras). This way a player will know exactly what slot is being used just by looking at the file name. Typically slot #101 is used.
The late release of the SGM utility is why there are so few Fan created Scenarios. Also, as Scenarios must must be given one of the only 28 available "slot numbers", and as each slot number MUST be unique (placing two SC4 files in the "Scenarios" directory that use the same slot numbers will crash the game until you remove the offending files), you are limited to the number of Fan made Scenarios you can keep simultiniously inside your "Scenarios" directory. There is no limitation on the other hand to the number of Saved Games you may have (as long as their SV4 file names differ that is).
Tip: Create a special storage directory to hold ALL of your Fan created Scenarios inside. Then, prior to starting RCT, move just ONE of these Scenarios into your "Scenarios" directory. When you are finished with this Scenario, simply move the file back out into the storage directory. In this manner, you will never have any problems with too many simulatnious Fan Scenarios or conflicting slot numbers.
This handy viewer utility called "RCT Explorer" allows you to view the parameters of a "Saved Game" or a "Scenario" without having to launch them inside RCT (i.e., park name, scenario goal, starting money, etceteras). This viewer is particularly useful for Fan created Scenarios, as it allow you to view the "slot number" used by the Scenario, and thereby prevent potential slot number conflicts.
Custom Scenarios can be just plain sandboxes, but the real advantage of being able to create custom scenarios is to actually create "Scenarios", with specific goals and limitations (just like the ones that came with the game). An example (available for the RCT2 game only unfortunately) would be the scenario I created for the 
UCES Project. A reading of the description for this scenario is a good lesson in how to make a Scenario exciting to play.