With <maximumascendingrate> the maximum ascent rate in metres per second (not metres per minute! — the conversion into other units is to be done by the processing software) is put into brackets as a real number.
The <maximumascendingrate> element stands inside <profile> (in so far as a calculation of an ascent profile is to be done), or <table> (if it's the maximum ascent rate to be used for table generation).