The Fit Tolerance is a feature that is utilized only if you are utilizing the 'Template' method of defining a Unit. A Template is created by clicking a single Spike within the distribution, and selecting 'Add Unit'. This spike acts as a central reference, and all other spikes that fall within the set fit tolerance of the reference spike will also be included in the unit:
You can increase or decrease the number of spikes in the unit, by changing the Fit Tolerance. A lower Fit Tolerance will reduce the number of spikes, and vice-versa:
The Fit Tolerance will be grayed-out if you create a unit using Contours (i.e. by manually drawing around the spikes you want in the unit, and then selecting 'Add Unit'). Since this method isn't using a single reference spike, the Fit Tolerance is not used (and hence it is grayed-out):
