Editing WoW:API Region SetPoint (section)

=== Edge definitions ===
The edge "points" (TOP, RIGHT, BOTTOM, LEFT) are not actual points, but... edges. To fully define a region position using edges alone, you will need to define all four (as opposed to using corners, where you only need two) edges.

Therefore, defining an edge does not necessarily place a region relative to the center of that edge (For example, if you define TOP and RIGHT, the region will be placed at the TOPRIGHT corner). However, by default a region is "pulled" to the center if the number of points defined is insufficient.

Example:
 Region:ClearAllPoints();
 Region:SetPoint("TOP", relativeframe);
 Region:SetWidth(100);
 Region:SetHeight(100);
will place ''Region'' to the top center of the ''relativeframe'' while adding a
 Region:SetPoint("RIGHT", relativeframe);
will place ''Region'' in the top right corner and now ''Region'' is fully defined.