You can mark a blank (open) slot as a spare.
There are 2 settings in Electrical Settings that allow you to specify whether the spare loads are included in the panel's Total Connected Load and Total Estimated Demand parameters (both for VA and Amps). The settings are Include Spares in Panel Totals and Run Calculations for Loads in Spaces.
Spares are locked by default, but can be unlocked. Locked spares do not move from their current slot or phase when you rebalance the panel schedule.
The term Spare displays in the Circuit Description column for all selected slots and the default phase values is 0. You can overwrite this value and assign phase load values to the spares. Loads assigned to spares have a 1.0 power factor.