expense

(1) A payment for an expense may be received by a volunteer for items such as uniform allowances or reimbursement for reasonable cleaning expenses or wear and tear on personal clothing worn while performing the volunteer work. Additionally, reimbursement for approximate out-of-pocket expenses for the cost of meals and transportation expenses may be made.

Source

24 CFR § 70.3

Scoping language

A volunteer, for purposes of this part, is an individual who performs service for a public or private entity for civic, charitable, or humanitarian reasons, without promise, expectation or receipt of compensation for services rendered, on a HUD-assisted or insured project which is subject to a requirement to pay prevailing wage rates.
(1) Individuals shall be considered volunteers only where their services are offered freely and without pressure and coercion, direct or implied, from an employer.
(2) An individual shall not be considered a volunteer if the individual is otherwise employed at any time in the construction or maintenance work for which the individual volunteers.