Each end of a line acts like a magnet and snaps the reticle to it if the reticle is close. You can start or end a new line on any end of an existing line by moving the reticle

close enough to the end of the line that the reticle snaps to it, then tap the
Add measurement plus sign

to either start or complete the line segment.
The perpendicular line tool appears whenever the first segment of a line is marked. This allows for easy measuring of height or width and helps ensure easy measuring of surfaces that might be difficult to measure, such as heights on a wall. To measure the height on a wall, mark a point on the floor, move the reticle

to follow the vertical line up until you reach the desired stopping point and tap the Add measurement plus sign

to end the line segment.