Usages allow you to compile a database of standards and assumptions. This allows your team to link the same data to multiple geometries within a project. From giving all the residential buildings the the same
buildCost to making all annotations
Giraffe comes with some default usages such as
Basement Carpark and
Annotation but you can create your own and copy them between projects. If you copy a geometry from one project to another, the usages will be copied too. See this example here.
The data model behind Giraffe is explained in more detail here.
Every geometry has a usage. That is how Giraffe knows what each geometry it is. Change a usage by selecting the geometry and changing usage.