From Ribbon Commander Documentation
rxExpand models element ST_Expand of the MS Custom UI XML specification which specifies the direction or directions that the relevant control expands within its container.
|rxeNone||Value not set|
|rxeHorizontal||Specifies that the control expands horizontally|
|rxeVertical||Specifies that the control expands vertically|
|rxeBoth||Specifies that the control expands both horizontally and vertically|
|rxeNeither||Specifies that the control does not expand|