"Frame within a FrameSet. The default is the first Frame added to the FrameSet when it is created (this Frame always has an index of 1).
When setting a new value for this attribute, a string may be supplied instead of an integer index. In this case a search is made within the FrameSet for a Frame that has its Domain attribute value equal to the supplied string (the comparison is case-insensitive). If found, the Frame is made the base Frame. Otherwise an error is reported.