There are 2 (Server) or 3 (Server) plugin managers loaded by REI at once. Each of them are responsible for different reloadables, and holds a different plugins list.
This is the base plugin manager which both client and server REI plugins are registered to.
This is the plugin manager handling client-specific reloadables which client REI plugins are registered to.
This is the plugin manager handling logical-server-specific reloadables which server REI plugins are registered to.
Instances of Plugin Manager can be accessed via methods in PluginManager, registering plugins are handled internally via PluginView, plugin implementations should look at Constructing Plugins instead.
PluginManager.areAnyReloading() can be used to check if there are any plugin managers currently reloading.
Individual plugin managers can be checked with PluginManager.getXXXInstance().isReloading().