@JoeX9977 not really random, but you should consider it random.
They use an algorithm to select stations within a certain distance of each other to form the “backbone” chain, which has to deal with ensuring their global RTK performance.
They will switch the stations each epoch and “randomly” choose stations that are eligible, however a station in a bad location might never be eligible depending on its relative position to other stations in the backbone.
They have not shared the backbone equation or selection logic algorithms, even with much complaint about it during the GIP process.
