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A mark may have one of two lifetimes: temporary or permanent. Permanent marks remain valid after arbitrary operations on the text; temporary marks do not. Temporary marks are used because less bookkeeping overhead is involved in their creation and use. If a temporary mark is used after the text it points to has been modified results will be unpredictable. Permanent marks continue to point between the same two characters regardless of insertions and deletions made before or after them.
There are two different kinds of permanent marks which differ only in their behavior when text is inserted at the position of the mark; text is inserted to the left of a left-inserting mark and to the right of right-inserting mark.