The Tiberian Sun Client is a complete, stand-alone Tiberian Sun package for both players and modders. It fixes all known (fixable) bugs in Tiberian Sun and offers a ton of new functionality, including a new easier-to-mod file structure, new game options and pre-defined starting locations and teams. With the Client's bugfixes and enhancements, Tiberian Sun players get a significantly enhanced experience compared to the original game, while enjoying gameplay exactly identical to the original game. For modders starting a new Tiberian Sun mod, it is not required, but highly recommended that you begin with installing the TS Client. The new file structure directly exposes the most critical INI and MIX files of the game, meaning that you don't need to download any special tools for most basic modding.
If you've been modding with the TS Client, you've likely often seen this window. It's because the updater of the client verifies files to make sure that your game version matches the official one for online play compatibility.
Now, it's quite annoying to always answer "No" to that dialog each time you want to enjoy your modded TS. So, with the latest update you can disable this functionality. Head to Resources\MainClient.ini after updating (don't forget to backup your Rules.ini, MIX files etc. changes before updating) and set the ModMode= key to true. After doing this the updater will be entirely disabled and you won't see the version mismatch dialog anymore. Your game version will show up as "N/A" for online players to make it clear that you're not running the official build.