I would say a person's time is not a resource as it is not property owned by the organization.
Per 2.2.2 in the constitution, A Circle’s Lead Link may define relative priorities for the Circle. In addition, the Lead Link may define a more general “Strategy” for the Circle, or multiple Strategies, which are heuristics that guide the Circle’s Roles in self-identifying priorities on an ongoing basis.
I suppose you could argue that "50% for role X and 50% for role Y" is a kind of strategy to guide priorities, however, keep in mind that this does not actually require a role-filler to allocate their time this way. It is still up to each individual role filler to decide how they are going to allocate their time across all the next actions and projects of all their roles.