In order to use label restrict, first, reload your page if you opened it before installing, or open a new page in the space, go to space admin menu, and click label restrict.
Click Activate, and add labels, every label in a different line.
Create or open a page inside the space, click the page bottom right button to add a label to this page, a popup window opens with the list of allowed labels, so you can only add labels from thoses defined in label restrict admin section of the space. Users will only add labels from thoses allowed by space admin. Select a label, click add, and close.
The goal is to restrict labels inside this space to thoses values : todo, draft, completed, sent to customer. So the users can only add labels from thoses values. So managers can know which are proposals still draft, or how much ones are completed, by clicking on the corresponding labels.
List of defined labels permet to restrict label input, so user can only add label to page from the ones in the list. User select a label in the list, Click Add, and Close.