When loading mmCIF structure files, the assembly setting controls whether PyMOL loads the asymmetric unit (assembly="") or a biological unit (e.g. assembly="1"). Assembly identifiers in mmCIF files can be arbitrary strings, but usually are numeric and most files define at least assembly "1".
New in PyMOL 1.7.8
Download the ASU and the first assembly, show them side by side.
set assembly, "" fetch 3bw1, asu, async=0 set assembly, 1 fetch 3bw1, assembly1, async=0 set grid_mode