See section 12.4.1 for points in space.
In the 2-d geometry screen in point mode, clicking on a point with the left mouse button will choose that point. Points chosen this are automatically named, first with A, then B, etc.
Alternatively, the point command chooses a point, where the
point (a,b) is specified by either the two coordinates a,b, a
list [a,b] of the coordinates, or the affix a + b*i.
(The marker used to indicate the point can be changed; see section 11.3.2.)
If the point command has two numbers for arguments, at least
one of which is complex but not real, then it will choose two points.
will choose two points named A; one with affix 1 and one with affix 2i.