Rockstar famously tests dozens of modes internally that never see the light of day. Build 350 contains a fully functional version of Stockpile (a Capture-the-Flag variant involving weapon pickups) with unique vehicle spawns that were later repurposed for Smuggler’s Run. The "126" version uses the original, un-nerfed weapon damage models.
If you ignore this warning and download the file anyway, you may face: gta v build 350 online 126 exclusive