BXL files typically contain one or more footprints, one or more symbols and one or more component definitions. Footprints can be loaded in pcb-rnd and sch-rnd can load the symbols. Component definitions are somewhat similar to sch-rnd's devmap concept but are not supported at the moment.
The first symbol from any BXL file can be loaded the usual ways in sch-rnd, e.g.:
Sch-rnd will load the first symbol from the BXL file. The only exception is when a subsymbol name is supplied to BufferLoad, e.g. BufferLoad(symbol, foo.bxl, bxl, bar) will load the symbol that's named "bar" inside foo.bxl. Subsymbol name is case sensitive.
Subsymbols of a BXL file can be explored by temporarily enabling the plugins/io_bxl/print_subsym_names config node and loading the file so that all subsymbol names are printed in the message log.