This is seen in some manifests/services for whatever reason. I can't find documentation for this value anywhere. It seems unused in official specifications as of right now. However, it seems in some services/places it is unofficially used as a PAL-version of BT-601 transfer, which makes sense.
Devine's code (and other services) wouldn't care about the difference here so currently it is just implemented as a remap from 5 to 6. In the future it may be changed and actually defined as two seperate BT_601 Transfer enum entries.