In plain English
FAR 52.204-12, Unique Entity Identifier Maintenance, is a contract clause prescribed at FAR 4.607(c), most recently dated Oct 2016. The complete official text is reproduced below, verbatim, from GSA's published FAR source files.
Its text contains no sentence requiring insertion into subcontracts; see the flowdown section below for what that does and does not mean.
Does it flow down to subcontracts?
No flowdown mandate found in the clause text
We scanned the full clause text and found no sentence directing the contractor to insert this clause into subcontracts. That is a statement about the text, not legal advice: a prime contractor may still flow terms down contractually, an agency supplement (DFARS, VAAR, …) may add requirements, and clauses listed below (if any) may order this clause into subcontracts from the outside.
Where it's prescribed
As prescribed in 4.607(c), insert the following clause:
Prescribing reference: FAR 4.607(c).
The official text, verbatim
As prescribed in 4.607(c), insert the following clause:
Unique Entity Identifier Maintenance (Oct 2016)
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(a) Definition. Unique entity identifier, as used in this clause, means a number or other identifier used to identify a specific commercial, nonprofit, or Government entity. See www.sam.gov for the designated entity for establishing unique entity identifiers.
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(b) The Contractor shall ensure that the unique entity identifier is maintained with the entity designated at the System for Award Management (SAM) for establishment of the unique entity identifier throughout the life of the contract. The Contractor shall communicate any change to the unique entity identifier to the Contracting Officer within 30 days after the change, so an appropriate modification can be issued to update the data on the contract. A change in the unique entity identifier does not necessarily require a novation be accomplished.
(End of clause)