How are these allocated? If I set these differently, will them be automatically ‘re-allocated’? 

These weights allow you to adjust the menu item position numerically and could be set in either of the formats below to achieve the same result. However, if you prefer visual representation editing the menu can be done in drag and drop format under the menu tile on the dashboard by pressing “list links” associated with the menu you would like to edit. (See academy video on menus for more info)