Faction Availability has no rules. It's a brawl between various conflicting sources, opinions, bizarre rituals and occasionally direct intervention by powers beyond the MUL team.
Usually we try to take all sources in to account. If there is a direct conflict (same unit, same time, different answers), then we often prioritize either a current Technical Readout or more recently published source.
But none of those are rules. Thirty years of sourcebooks and dozens if not hundreds of different authors means we can write specific rules, but have to take each case as it comes. You are welcome to make suggestions, that's why this errata thread specifically allows suggestions. We don't require a definitive answer before you ask, because there often isn't one.