The slotnumber corresponds to the slot number. The colon after the slot number is required. For example, if a device has 4 slots then there would be slotdef=1:..., slotdef=2:..., slotdef=3:..., and slotdef=4:... attributes somewhere in the symbol and be made invisible. This is a free standing or floating attribute.
The #'s have a one-to-one correspondence to the pinseq attributes and specify which pinnumber=# is used during display (gschem) or netlisting (gnetlist).
It is recommended that all symbols which have slots have a slot=1 (5.2) attribute attached in the same fashion as the device= (3.1) attribute.
See 7400-1.sym as a concrete example.
Examples: slotdef=1:1,2,3 slotdef=2:4,5,6 slotdef=3:7,8,9
This attribute replaces the obsolete slot#=# attribute.