As programmers may know, a "global variable" is a variable all scripts have access to, therefore its effects can be carried over to other Save Files. In plain English: Effects you can keep after missions. In this topic, I will try to document as many of these variables as I can, along with their in-game effects.
Marked virus (only confirmed on PSP): The "marked virus" does two things: Makes your vehicles act as mission-important, whereas they're not, and allows any vehicle owned by script to be stored in garages on the spot, be it vehicles converted to mission-important by the garages themselves, or vehicles from actual missions. This virus - in the form of opcodes 01CB and 0677 - is stored globally in garage slots, and can be acquired by obtaining either the Avenger from "Avenging Angels" or the Landstalker from "See the Sight Before Your Flight", storing them in the garage along three other vehicles, and keep opening and closing the garage until every slot is infected. This virus can be transferred over to the rest of your Save Files. After hard resetting, if no infected vehicles are present within the Save File, the marked virus will be gone, so keep that in mind.
Working drawbridge (confirmed on all platforms): This variable makes the drawbridge that connects Staunton Island to Shoreside Vale functional before completing the mission "Love on the Rocks", despite the protesters being present. To activate this, just start the mission "See the Sight Before Your Flight", but instead of passing or failing it, load a Save File of choice. The bridge will now be open. This effects goes away after hard resetting, but can be replicated anytime.
Dead traffic in Wichita Gardens, half of Porter Tunnel, and the airport (currently only confirmed on PSP): This variable disables traffic around Wichita Gardens (the block where the mission "Rough Justice" takes place), the part of Porter Tunnel that passes underneath and the Francis International Airport, as well as the road that connects the airport to the main city of Shoreside Vale, and is associated with "AWOL Angel". Start "AWOL Angel", but instead of cancelling the side mission by any means, load another Save File. Traffic density around these areas is now set to 0. The effect goes away after hard resetting or starting "AWOL Angel" again and cancelling it. For some reason, after doing the latter, not only does traffic come back, but is also enabled inside the airport.
Dead traffic in Fort Staunton: After completing the mission "Bringing the House Down", traffic around the destroyed part of Fort Staunton will be disabled. Now, if you load a Save File that has "Bringing the House Down" completed and load another one before this mission, Fort Staunton will obviously be intact, but traffic will remain disabled! The effect goes away after hard resetting, but this is only true for Save Files that do not have "Bringing the House Down" completed. If Fort Staunton is destroyed, traffic is permanently disabled within the Save File.
There are other variables in the game, such as the ability to gain a 6-star wanted level before unlocking Shoreside Vale with "AWOL Angel", but these are local; they are only true for the current Save File you're playing, therefore don't affect other ones.