Please Refer to Project Stages (Milestones)

At the commencement of any Project, Management logically breaks up a project into logical stages to control construction.
These stages could be floors, buildings, etc or defined end user spaces, thus the structural stages could be different to fit-out.

Where BIM (Building Information Model) is utilised, the stages should logically align with the BIM model.

Therefore when a subcontract is created, it would logically be broken down by the defined stages now called "Milestones".