H5PL_GET_LOADING_STATE retrieves the bitmask that controls whether certain classes of plugins (e.g.: filters, VOL drivers) will be loaded by the library.
Types of plugins that are subject to programmatic control are listed in the definition of the
H5PL_type_t ENUM. That list is presented in the “Plugin Type/Bit Position” table in the H5PL_SET_LOADING_STATE reference manual entry.