Difference between Frame Rectangle and Alignment Rectangle

Question

Can anyone explain what is the difference between Frame Rectangle and Alignment Rectangle in the size inspector?

Answer 1

From Apple's old Auto Layout Guide:

Layout Operates on Alignment Rectangles, Not on Frames

When considering layout, keep in mind that the frame of a control is less important than its visual extent. As a result, ornaments such as shadows and engraving lines should typically be ignored for the purposes of layout. Interface Builder ignores them when positioning views on the canvas—in the following example, the Aqua guide (the dashed blue line) aligns with the visual extent of the button, not with the button’s frame (the solid blue rectangle).

To allow layout based on the presentation of the content rather than the frame, views provide an alignment rectangle, which is what the layout actually operates on. To determine whether your override is correct on OS X, you can set the NSViewShowAlignmentRects default to YES to draw the alignment rects.

So, that pop-up menu in Xcode is providing you the ability to view and edit a view's size and position based on its alignment rect or its frame rectangle.