The definition in A-21 of "allocable" states that:

A cost is "allocable" to a sponsored agreement if the goods or services are
chargeable or assignable to that agreement in accordance with relative
benefits received or other equitable relationship. A cost is allocable of
it is incurred solely to advance the work under the agreement; if it
benefits both the agreement and other work of the institution in
proportions that can be approximated through use of "reasonable methods";
or, where it is necessary for the overall operation of the institution and,
in light of the principles in A-21, is "deemed to be assignable in part" to
sponsored projects.

That is straight out of A-21 and seems reasonably plain for a concept that
is not always simple to explain (compared to allowability, for instance).
You may want to stick with the A-21 definition.

James F. Ball
Sponsored Program Officer
The Ohio State University Research Foundation
1960 Kenny Road
Columbus, Ohio 43210
614-292-6278
614-292-4315 Fax