Just going out on a limb here...
Are you talking about the way that individual drawings can be set to use either CTB or STB files? When the drawing is set to one type, the dialog boxes will not even acknowledge the existence of plot tables of the other type. In other words, you only see .CTB files if your drawing is set to use CTBs, and vice-versa with STBs.
The only way I know of to change from one to the other is with the incredibly-clunky and mis-implemented CONVERTPSTYLES command.