The musnad is the predicate: the act, condition or attribute asserted about the musnad ilayh. It may be a verb, the predicate of a mubtadaʾ, or, in special constructions, a descriptive mubtadaʾ that takes a raised complement in place of a separate predicate.
“سَافَرَ مُحَمَّدٌ”
“Muhammad travelled.”
“سَافَرَ” is musnad; “مُحَمَّدٌ” is musnad ilayh.
“عَلِيٌّ مُقِيمٌ”
“Ali is resident.”
“مُقِيمٌ” is the predicate and musnad; “عَلِيٌّ” is musnad ilayh.
In the special pattern “مَا قَائِمٌ الزَّيْدَانِ”, the descriptive term can function as a mubtadaʾ with its raised complement supplying what would otherwise be a separate predicate. This is a special syntactic case, not the usual noun-plus-predicate construction.