AT&T MERLIN LEGEND Communications System PBX System Planning Manual page 165

Modifications
step 5
5-4 Adding to the System
Placing additional modules in the control unit calls for a recalculation of unit
loads. See System Reference for instructions.
If the control unit has an available slot to the right, and the guidelines permit
placement there, continue to step 5.
If you must rearrange existing modules to accommodate the new one,
continue to step 6.
Revise or complete the required PBX System Planning Form(s) according to the
directions shown in Tables 5-2,5-3, and 5-4.