A click with the Selection tool on a guide selected with objects makes
it the key (select.key {guide}), drawn thicker; a click on it again
cancels it. Align to Key Object is on with one object and a selected
guide and makes the guide the key. object.align lines the objects (or
selected anchors) up on the guide's line and leaves the guide where it
is: a vertical guide sets x only, a horizontal one y. The selection
keeps key_guide beside key, never both; it goes with the guide or the
objects, and follows the guide when earlier guides are deleted.
Distribute Spacing doesn't space from a key guide.