The term directionality often refers to two distinct directions in a writing system.
Most commonly directionality is used to refer to the direction in which individual characters are written in a line. For example, the directionality of latin script is left-to-right, the directionality of arabic script is right-to-left, mongolian or asian script is top-to-bottom, etc. Let's call this more precisely line direction / orientation.
However another direction is required to define a writing system, namely in which direction the lines progress. This "direction of line progression" is perpendicular to the line direction, which in a 2D plane means it can have two possible values. For example, in latin script the lines progress from top to bottom. But you could imagine a script where the lines start at the bottom and then stack upwards. And indeed, there are scripts with vertical line direction that differ in the "direction of line progression". For example, most asian script progresses right-to-left, while mongolian script progresses left-to-right.
It's this "direction of line progression" that's usually conflated with "directionality" and which I try to untangle. Something along the lines of line ordering, line progression direction, line progression order maybe?
For reference, this question uses "directionality" to refer to both of these example:
Wikipedia use the syntax
right-to-left, top-to-bottom to describe the two directions on Right-to-left scripts. However they nowhere mention any terms for each. Unfortunately they also propose to shorten it to the first direction only,
(commonly shortened to right to left or abbreviated RTL)
which completely disregards that the second can still have two possible values (2 perpendicular directions on a 2D plane).