First of all, a tiering system is the usage of tiers like OU, UU, RU, etc. Most games are using a usage-based system where each month ladder activity and usage determined if a Pokémon drops or rises from a tier to another. In a VR-based system,
like LGPE metagames, a Pokémon rise or drop is determined by its ranking in the Viability Ranking lists, that are established on a regular basis (after any important even in practice) by a qualified playerbase and/or councils, following one or multiple thresholds.
In LGPE the cutoff is B rank. For example, if Melmetal that is currently OU would be ranked C after a VR, it would drop to UU. In the opposite way, if Mega-Pidgeot that is UU would be ranked A after a VR, it would rise to OU.