Sets an object to be positioned and sized exactly the same as another object.

MyObject:SetAllPoints(relativeRegion or "regionName") 

... is exactly equivalent to:

MyObject:SetPoint("TOPLEFT", relativeRegion ,"TOPLEFT", 0, 0)
MyObject:SetPoint("BOTTOMRIGHT", relativeRegion ,"BOTTOMRIGHT", 0, 0)

• See Region:SetPoint()
• region may be given as nil to anchor relative to the whole screen.
• region unspecified anchors the object to the parent.