As with any clause, we look at the prescription to see the applicability of when it has to be added to a contract. FAR 22.1006(b) provides the prescription for clause 52.222-42: 

The contracting officer shall insert the clause at 52.222-42, Statement of Equivalent Rates for Federal Hires, in solicitations and contracts if the contract amount is expected to be over $2,500 and the Service Contract Labor Standards statute is applicable. (See 22.1016.)

Given there are nuances to the applicability of the Service Contract Labor Standards statute, refer to FAR subpart 22.10 to see how it applies to your acquisition. Each agency/service typically has labor advisors, so you can also refer to your respective labor advisor; see below link. 

https://www.acq.osd.mil/dpap/cpic/cp/service_labor_advisors.html