Hi @BenLewis,
basically when adding a Fictitious Clearance to a contact surface, we add a layer of a fictitious material (with no additional stiffness) with the defined clearance as thickness. On an algorithmic point of view, when it comes down to compute the (signed) contact distance, the solver removes from the actual geometrical distance the fictitious clearance and the result will be used to evaluate the contact state (open/closed) and the contact forces.
So, if your geometry originally has a gap of 2mm and you apply a fictitious clearance of 1mm, the solver would compute a remaining gap of 1mm. After moving one part for 1mm the contact will become active and contact forces will start acting. Using an exact contact algorithm (Lagrangian), the gap will never be smaller than 1mm.
Does this explanation makes sense to you?
Best,
Richard