What is the correct return value for valid in the geometry callback?

[Raildex]

In Embree3, i needed to return 0 when the hit "needs to be rejected". In the Changelog for Embree4, the docs say I need to return -1 when the "hit was not found" and 0 when it was.

The manpage says the following:

The valid parameter of that structure points to an integer valid mask (0 means invalid and -1 means valid)

What does valid/invalid mean in the first place?