This is part of the printing support in CSS. The property can accept either the auto keyword, which allows the element to be printed on the current page. It can also have a named page layout cited as an identifier value. That named layout must have been created earlier in the style sheet.
When this style is deployed, one of two possibilities pertains. It names a page layout which is the current layout being rendered or some other layout that is currently being written on. If the identifier corresponds to the layout currently being rendered, then the element is simply added to the page unless the page is full and then a new page is created in the normal way. If the page currently being rendered is of another type, that is its layout is different, then a new page is started and the element is the first item placed on it.