Two treatments in one appointment. A PCA chemical peel followed by an enzymatic therapy, so the peel does the resurfacing and the enzyme step refines and calms on top of it.
The reason to combine them is that they work on different things. The peel loosens the deeper layer of dead cells and drives turnover; the enzyme step clears the surface and adds hydration back, which takes the edge off the recovery. PCA positions the combination for texture, dullness and uneven tone with minimal downtime.
This is the choice when a single peel has not moved things as far as you wanted, and you would rather get more out of one visit than book two.