The attribute value should consist of three character, each of which must be either "U" or "L". The first character in the string specifies the position of the corner on the first graphics axis. If the character is "U" then the corner is at the upper bound on the first graphics axis. If it is "L", then the corner is at the lower bound on the first axis. Likewise, the second and third characters in the string specify the location of the corner on the second and third graphics axes.
For instance, corner "LLL" is the corner that is at the lower bound on all three graphics axes, and corner "ULU" is at the upper bound on axes 1 and 3 but at the lower bound on axis 2.
The default value is "LLL".
AST A Library for Handling World Coordinate Systems in Astronomy