Each substituend SN, it seems, would amount to the same thing as its corresponding UN in SOME not very strained context.

Note that a variable is a substituend of a variable only in a vacuous sense , and so we shall be ignoring them while speaking of the substituends of a given variable.

The pragmatic process of substitution requires some given context to provide an appropriate substituend for a metavariable projected by a pronoun.

[…]but since the substitute y is not treated like the substituend vowel, the option will not prevail.

The substituend item bhis is one of the twenty-one case suffixes (sUP or vibhakti) introduced as a set in Pāṇ. 4.1.2.

But those (same) meanings belong to the substitutes because of the rule accepted in the Bhāṣya that only that is a substitute which is capable of expressing the meaning of the substituend.

Substituend

Meaning

Noun