Phases are an optional component of a project, and are for managing projects with large numbers of milestones.
The project's "schedule" tab and the calendar's gantt chart are the only two screens in the software that filter events by phase.
A project can have any number of phases, but phases may not themselves have phases. Like projects, phases don't have any date attributes themselves, but take their start and end dates on the gantt chart from the earliest and last dates of the events associated with that phase.
A couple of other things to note: phase names must be uniques within a project, and deleting a phase deletes all the milestones associated with that phase.
How are Phases created?
There is a "new phase" button on the project's "schedule" tab, but phases may also be created when a milestone template is applied to a project. If there is a phase name associated with a template's milestone record, a new phase with that name will be created for the project when the template is applied. If a phase of that name already exists for the project, then the existing phase will be used for milestones whose phase has the same name.
Note that only on the Schedule tab of the Projects layout may you change which phase a milestone belongs to.
Are "Phases" records in their own table?
Yes. There is a simple table of phases in the Data file. Phases are linked to projects and to events with cascading deletes set up both ways, so if you delete a project, all its phases are deleted, and if you delete a phase, all its events are deleted.