For a release, you can add a feature to a phase to enable an understanding of how far through a release a feature is. The phases defined are relevant to that release.
A 'product' release can be attached to a master release. This is likely to be a larger programme of work. The master release will have multiple product releases attached to it.
The master release will have different phases and milestones defined to the phases and milestones defined for a release.
In a mater release it is not currently possible to attach any of the features within a release, to a phase within a master release. This is fine. The logic appears to assume features will be created for the master release, but this is not technically possible. What makes more sense from a human process is that the programme manager is able to attach the release to a phase within a master release.
This is a key aspect of being able to report progress clearly for a master release.