To set up a sub limit, first set up the standard limit.  Then go to ELCYLimit, Limit, Create Limit and select the radio button Sub Limit, then choose the Standard Limit that this is to be a sub limit of. 

The rest of the limit set up is similar, please refer here.

However if the main limit does not permit something (e.g. no SBLCs) then the sub limit will not permit it either.