  The paper concerns foundations of sensitivity and stability analysis, being
primarily addressed constrained systems. We consider general models, which are
described by multifunctions between Banach spaces and concentrate on
characterizing their well-posedness properties that revolve around Lipschitz
stability and metric regularity relative to sets. The enhanced relative
well-posedness concepts allow us, in contrast to their standard counterparts,
encompassing various classes of constrained systems. Invoking tools of
variational analysis and generalized differentiation, we introduce new robust
notions of relative coderivatives. The novel machinery of variational analysis
leads us to establishing complete characterizations of the relative
well-posedness properties with further applications to stability of affine
variational inequalities. Most of the obtained results valid in general
infinite-dimensional settings are also new in finite dimensions.
