Two or more points are horizontal if they lie on a line parallel to the x-axis. There is no indicator for the horizontal points relation.
To detect a horizontal points relation implicitly
The points are snapped as horizontal.
note: A horizontal points relation can be detected implicitly if the Horizontal points check box in the Tools | Options | Relations tab is selected.
To detect horizontal points relation through snapping
note: This action does not involve the setting of such a relation but makes the point horizontal with the reference point.
Example
Two horizontal points
note: Since a horizontal points relation is defined between two points and the x-axis, when you move the horizontal points they are not kept parallel to x-axis: the relation is detected, but is never set.