When working on a project in Teamwork project, with multiply users, the project is not stored on your hard drive but it is "downloded" from a BIM Server (which usually is a second computer in the office were the BIM Server is installed). So to work one logs into the project, it is dowloaded and You see it in Archicad but You can't edit anything until You select this object and press "Reserve" button in the Teamwork palette.
You also reserve setting dialog, one of them is the Surface Dialog with materials.
If you are work on a standard project (standalone) in AC you will not have those Reserve buttons.