Here in our general documentation page you can read about the things that are common for most of the methods and objects of the API. It is good to read through all of the material here before you start working with the API.
Some attributes are nullable which is indicated by a question mark (?) after the data type. This means that the value "null" can be assigned to them even if you usually cannot assign null to these primitive data types.
The Brickid is the internal id for objects in the BFS system. It is assigned internally and cannot be assigned from the outside of the application.
The Brickid is present as input for some of the create methods but is never used in these cases. It is only used for filtering in get- and update methods since you cannot assign a BrickId yourself. If you want to keep track of the BrickId of the object you create, it is returned in the entities in the response.