The difference between relDir and parentID

Do you mind explaining the difference between `relDir` and `parentID`?

As an experiment, I replaced all references of parentID with `relDir` in the library. I then updated all tests that relied on `parentID` to use `relDir` instead. All tests are passing, which makes me think that it is a duplicate property.

Thanks!

Inspect the amd-resolve library's definitions and usages of relDir and parentID, then review the tests that cover both properties. Determine whether they represent distinct concepts or duplicate behavior, and document the difference with a focused explanation or test coverage.

Written by the indexing model from the issue text.