At the beginning and the end of the project one works in the abstraction, whereas in the middle of the project one works in the concrete world.
And this on the 3 axes of work (WBS): Activities, Products, and Zones.
For example, on a construction project of a Power plant, at the beginning and the end of the project the Zones are functional, one call-that “Process units” or “Working units”; then in the middle of the project, at the time of construction they are geographical, which is more concrete. For the Activities it is similar, to design and to test, it is abstract like connotation, whereas to install and to build it is concrete. And for Products, it is the same, one studies and one tests by functional systems, which exist not physically, whereas one buys and one installs equipment, which is quite concrete, them.
Then, work is abstract in the beginning and end of project, and it is concrete in the middle.
