Learn English – Noun to use for a person who has commissioned work from a contractor

Your question is somewhat misleading, and may contain a false assumption. A statement like the following:

Customers who have never paid a dime

is not a sentence. It's a noun phrase. Let's break that down:

Customers is a noun. Nouns are not inherently subjects or objects, so it's not correct to refer to this as either the subject or the object.

The remaining part who have never paid a dime is a relative clause, which is an embedded sentence that is used to modify something in the larger context. The subject of the embedded clause is the relative pronoun who, and the remainder have never paid a dime is the predicate of the relative clause.

However, using terms such as "predicate" and "relative clause" may be intimidating to people using your software. My best offering for newbie-friendly terminology would be:

• Call Customers a noun (most people at least know what that means)
• Call who a "question word", and make it clear by example what you're referring to
• Call have never paid a dime a "description"

Since the restrictive adjective is indeed describing a restriction, the lack of a restriction should be most appropriate.

Pablo's art is eye-catching, especially his commissioned work for MOMA.