The Center Tag will center the text of the current line. It does not work with an entire paragraph -- it is designed for headlines you wish to be centered. The Page Break command forces a new page to begin on the line where you put the tag. Both of these tags are only valid at the beginning of a line; the menu commands are disabled when your text cursor is anywhere else. If you manually type them in elsewhere, they will show up as text within your document.
Just like with Bookmark codes, Center and Page Break tags are removed when your document is exported.