I am examining some of the FAA's SID charts and, as far as I can deduce, each SID provides information concerning the altitude when it is necessary and, if not changed by the controller, each transition/route description will end with something such as "Maintain X altitude etc." which seems that it is the same as the Top Altitude.
Since this information is written in SID's textual description as "Maintain X altitude", why is there a need to exist also separately as Top Altitude?