Backtracking semantics unclear in delegated roles

I'm trying to wrap my head around the whole delegated roles part of the spec. I see the possible use cases for delegating the authority of parts of a repository to different entities outside the main repository authority.

What I'm wondering about is the use of the backtrack flag: as I understand it, it states that "files that are not signed by this role might be provided by other (trusted) roles, go look in other roles as well." As clients should always check all target roles to resolve a particular target file using some sort of priority scheme this flag serves no use.

It might even be harmful: consider that all target files are mapped to only a single target role, with backtrack = false, this could lead to a situation that when another target role (purportedly or intentionally) lists the same target file the client can see different truths depending on the order in which it considers roles. Thus it has to specify a priority scheme and therefore consider all roles in order...

No file, test, or entry point is named. Start by resolving the intended semantics of delegated roles and the backtrack flag in the specification; done means the specification clearly states whether backtracking is required and how conflicting target roles are prioritized.