It's a separate grid that's managed privately but overseen by tex gov with a lot less strict regulation than the national grids.
It seemed great at first because the prices were really cheap, but the lack of good oversight meant the winterization didn't cover the 20 year horizon let alone a 50 or 100 year one. Temps got unusually cold in 2021 which led to huge power outages, deaths and incredible price problems, up to 180x[0].