Some teachers may not want to see all of the subjects that are available across the entire school and they want to create a list of 'favorite subjects' only. Teacher Dashboard has a Subject Manager where individual teachers can show or hide subjects from the global school wide list.
Login as a teacher and go to your subjects page.
From here you can click on 'show' or 'hide' to pick which subjects you want to display in your subject dropdown when using quick share or setting an assignment.
As subjects are just used to manage the filing system, you can create some administrative subjects for when sharing files and assignments between teachers. E.g. Reports.