If the input Deploy With Utilisation = Yes, STEM will allow for the same fixed overhead at each site. Otherwise, it is only applied as a single overhead on the overall demand.
The figure below compares the results for two scenario variants, Lean and Fixed NEW, which switch the Minimum Slack Capacity between 0 and 15 ports, respectively, in a situation where the overall demand gradually increases from zero to 1000 subscribers over ten years (with a Monte Carlo distribution over 10 sites).
Additional fixed overhead with Minimum Slack Capacity when Deploy With Utilisation = Yes
The deployment calculation allows for the notional slack capacity at each site to vary in the range 15–65 ports (average of 80% of a unit) rather than the unconstrained 0–50 ports (average of 50%), resulting in an extra 10 × 30% slack capacity (equivalent to three more units; i.e., the 150 subscriber difference you can see on the chart).
See also 10.3.25.2 Minimum Slack Capacity.