suppletive

Making up for deficiencies; supplementary; suppletory.

Supplying an etymologically unrelated word with forms, or which is used as one of its forms, by suppletion (for example, in English, better and best are suppletive forms of good).

... of optimus, meaning “best,” which in turn is the superlative suppletive form of bonus, “good.”

Relating to laws which can apply in a contract if the parties choose to agree to this.

Article 11 of the Louisiana Civil Code provides the legal framework for the doctrinal distinction between imperative, or mandatory, and suppletive laws. Although neither term is included in the article, traditional civilian doctrine characterizes as imperative those legal precepts rooted in public policy which may not be set aside by private agreement. Suppletive laws, on the other hand, are those legal norms designed to supplement the parties' will in cases wherein its application is not excluded.

...I deem little short of frivolous the contention that the Equal Protection Clause prohibits enforcement of marital obligations, in either the mandatory or the suppletive form.

A form used by way of suppletion.