A Phase is a subcategory of the overall job and can be used for both calculating costs or for billing. Phase subtotals a group of related Cost Activities. If a Job is a combination of sub-jobs for cost and income purposes, then you should set up several Phases (Cost and Income) for a Job.
For example, if you are building an office complex and you want to keep track of costs for the excavation, the foundation, and the framing, you would set up three Phases. The Phases could be identified as EXC, FND, and FRA. Phase provides a method of billing during the construction period that allows you to bill your customer when each of these Cost Activities is completed.
If you want to calculate costs for the Job as a whole, then you might only assign one Phase to that Job or not use this distinction at all.
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