NEC Express5800/120Rh-2 N8100-1126F - Manual (Page 82)

3-8 Setting Up Your Server
Only the inner rails remain screwed to the server when the rail assemblies have been removed.
IMPORTANT: The removed rail assemblies are to be installed on the
inner rails later. To install each rail assembly on the correct inner rail,
make a mark on the assemblies. Similarly, when installing more than
one server, distinguish between the pairs of inner rails and rail
assemblies of the servers by making marks.
Inner rails