Initial Spares

Looking for some guidance from the collective wisdom and experience of the group.

I can find information on what initial spares are, but I am having difficulty finding information on how we contract for them.

My understanding is that the full funding policy requires funding of initial spares (i.e., initial spares are purchased with the end items). What has been your experience in defining the requirement for initial spares and the contract type that is used? For example, does the contract define all the parts that are required (6 widgets, 3 gizmos, and a partridge in a pear tree), or does the contract allow flexibility? Is delivery required to be made with the end item, or can it be later? What pricing structure is used?

TIA.