You have learned that when writing your tracks, you will be operating within a particular key. The key you are using will be identified by the note that is being used as the tonic. For example, if you are writing in the key of Bb major, you will be using the note Bb as the tonic. For the key of F major, you will use the note F as the chosen tonic. These are differences of key, and the more keys you know, the more choice and freedom you will have when writing your music.