UIView documentation, Apple describes alignment rects:
The constraint-based layout system uses alignment rectangles to align views, rather than their frame. This allows custom views to be aligned based on the location of their content while still having a frame that encompasses any ornamentation they need to draw around their content, such as shadows or reflections.
At first glance, this feature feels unnecessary: views can draw outside their bounds seemingly without performance issues1. However, the differentiation between frame and alignment is a powerful, and easily overlooked, feature in Auto Layout.