I'm more familiar with those terms in CFD (computational fluid dynamics) modelling. In that world, slip boundaries are those where no viscous drag occurs (e.g. a symmetry boundary), no-slip conditions are applied where viscous terms are calculated at the "walls".
I suppose a similar boundary could exist in FEA, slip being a condition where only normal forces are transferred across the boundary (i.e. displacements along the boundary are allowed), no-slip being where no displacements, either normal or along the boundary are allowed.
