This page aims to give a full explanation on vehicle flags, including what they do, how they work, and why the advanced versions of them cause such consternation within the community. There's also this playlist for a journey of discovery about advanced handling flags in particular.
Written by: Broughy1322
Vehicle flags, or just flags, are essentially attributes that can be toggled on or off on a per vehicle basis. In GTA 5 they come in four different varieties:
Each of these varieties has 32 flags associated with it. Each flag is responsible for a certain behaviour and can be turned on or off for any vehicle by a string of numbers in the vehicle's handling data. An example is the tyres_can_clip handling flag which, when turned on, allows cars to have a smoother ride over bumps in the road when tyres with larger sidewalls are applied (like off-road wheels). This flag always does this function and always in the same way, meaning it's not possible to change the behaviour for a single car. It's either on or off - there's no half-way or partial effects.
Within the handling data for every vehicle there are four lines with an associated string of numbers. These four data points (strModelFlags, strHandlingFlags, strDamageFlags, and strAdvancedFlags) correspond to the varieties of flags mentioned above, and the numbers associated with them tell the game which individual flags are applied to any given vehicle.
Each of the 8 positions (digits) in the flag's associated string of numbers represents 4 different flags, for a total of 32 toggleable options. The 4 different flags for each position can be activated by the value being set to either 1, 2, 4 or 8, with a value of 0 meaning the flag is turned off. Other numbers (1-9) and letters (A-F) can be used to activate multiple flags for each digit in the following way:
reduced_mod_mass handling flag it needs to have the 2nd flag in position 1 activated, counting right to left. This would be written as 00000002.has_kers handling flag it needs to have the 3rd flag in position 1 activated, counting right to left. This would be written as 00000004.tyres_can_clip handling flag it needs to have the 2nd flag in position 5 activated, counting right to left. This would be written as 00020000.tyres_can_clip flag and the 6 activates both the reduced_mod_mass and has_kers flags.The above example would be written in the handling data for the car as <strHandlingFlags>20006</strHandlingFlags> since preceeding zeros aren't counted.
Given there are 4 different varieties of flags (model, handling, damage, and advanced), each with 32 individual flags that can be toggled on or off, understanding what all 128 flags do can be difficult. Nevertheless the community has done its best to determine the interal names of as many flags as possible, alongside working to understand how they affect vehicle behaviour when applied.
All known flags and their functions can be found within the metadata glossary page of this site, alongside which value activates them in the handling file string. All model and damage flag names are accurate to what Rockstar use themselves, as are almost all handling flag names, after being datamined from files left over in Red Dead Redemption 2. If a flag's name is preceeded by an underscore on the glossary then it is not an official name.
Going into great detail about advanced flags is beyond the scope of this article, however it is enough to say that their addition to the game has not been met with much enthusiasm by the general community.
Advanced flags appear to be a way for Rockstar to experiment with different car behaviours outside the usual model, handling, and damage varieties, as new advanced flags crop up from time to time when vehicles are added to the game with them activated. Unfortunately for players, Rockstar's experiments aren't always successful. Their inclusion on cars often ended up leading to undesirable behaviour that made them worse driving experiences across the board when compared with how the cars handled with the flags deactivated (especially in the early days of their introduction).
Advanced flags have continued to be used to experiment with different behaviours and fixes over the years and, while they've never been removed from cars, their overall effects have sometimes changed across the board to make them less drastic. For a more detailed story "as it happened" see this playlist.
Readers of this article should now be able to use the information here, alongside that listed on the metadata glossary, to decode which flags are applied to which vehicles in GTA5 based on their handling file data alone. However GTACars.net does the hard work already since the names of each flag that are activated for each vehicle are listed on its respective vehicle page. Simply cross-referencing those names with the metadata glossary will allow the reader to see what effect that flag is having on the vehicle, and the site will continue to be updated as more information is discovered. As a side note, you can also use this online tool to decode or modify flags from the handling files.