First of all, 'shall' is still very much in use in our federal contracting lexicon. IAW FAR 2.101: "Shall" is defined as - "means the imperative"; "Must" is defined as - (see "shall"), in these cases, both 'shall' & 'must' are imperatives. The word 'imperative' is not defined by the FAR, so we seek guidance in the dictionary, which describes an imperative as, "An authoritative command".

Their actual usage in contracting is subject to the discretion of the KO, policy office, and legal representatives. Generally, the more established the individual, the more likely they are to prefer to use the word 'shall'. The use of 'shall' is borne out of it's definition from "Blacks Law Dictionary" prescribing it as an imperative and it's frequent usage in precedent setting 'case law'.

The usage of the word "must" is increasing, but has not supplanted 'shall' in federal contracting circles.

This is not prescriptive, but often KOs will use the term 'shall' in the contract and its appplicable clauses, while using 'must' more often in Statements of Work, Performance Work Statements, and Statements of Objectives.

Both words are imperative and describe what one party or more parties to the contract are required to perform.