Subphase

Subphase: A subphase is used when there are phases that need to be subdivided. This might occur in a large number of scenarios, for example if the phase represents a task that Project Management has decided can be separated into a number of smaller tasks, the subphase will represent the individual smaller tasks. These need not be divided evenly for some may be larger than others, nevertheless, in many scenarios it will be expected that subphases are equal to one another.
A phase in the context of Project Management is any distinct time period in a sequence of events. A subphase is thus the smaller time period available for a portion of a sequence of events to be achieved. Alternativel, a phase may also represent a stage that must be reached, while a task leading to the stage is known as a subphase. In such a case, Project Management may evaluate how much time each task will take to finish, if all tasks together enable the stage to be reached. Alternatively, Project Management may determine the total amount of time allowed for the entire stage, and subdivide the allowed time amongst all of the tasks, to enable the stage to be completed on schedule. More personel may be needed if the allowed time per task is less than the evaluated task time to completion.

This term is defined in the 3rd and the 4th edition of the PMBOK.