The variable overhead required by a Maximum Utilisation constraint makes most sense for a continuously variable demand like bandwidth where some operational headroom is often left to allow for temporal fluctuations in demand. In contrast, the fixed overhead of Minimum Slack Capacity is more likely to be used for a discrete capability such as ports where it is common to leave some capacity free to minimise delay when adding new customers. Therefore, it is unlikely in practice that the two effects will be required for the same resource.
However, it is not an error to specify both inputs in parallel and the resulting calculation is additive: STEM installs sufficient capacity to allow for the specified minimum slack in addition to the variable overhead on top of the basic demand. In the single site case, the capacity installed for a demand, d, will be at least d/MU + MSC. STEM does not apply the maximum utilisation overhead to the minimum slack part as this is not required if the minimum slack represents spares which are available to swap with other used parts.
Alternatively, if the minimum slack is capacity reserved for adding new customers, then an inflated capacity would have to be reserved to allow for the usual maximum utilisation to hold if/when those customers are added. Therefore, if CB is the intended customer buffer, then you should consider setting the input Minimum Slack Capacity = CB/MU. The resulting capacity installed at a single site will be at least d/MU + CB/MU = (d + CB)/MU.
For a resource with inputs Capacity = 50, Maximum Utilisation = 80% and Minimum Slack Capacity = 15, the combined effect is that the next unit will be installed when the demand exceeds 28 (= 35 × 0.8), 68, 108, etc., rather than the usual 50, 100, 150. This allows for at least 15 spares, or the potential to add 12 (= 15 × 0.8) customers without having to install an extra unit.