In order to make sure two objects are perfectly vertically aligned, a user should drag one of the objects next to the other until a guide appears

In order to make sure two objects are perfectly vertically aligned, a user should drag one of the objects next to the other until a Smart Guide appears.

Smart Guides are temporary snap‑to guides used in the  vector graphics editor Adobe Illustrator. Smart Guides appear when you create or manipulate objects or artboards.

answer; peter zenger;