Index may be any integer value. Access to Bookmarks property looks for a TBookmark object in the BookmarkObj collection that has specified index (TBookmark.BmIndex). 

Reading: 

If bookmark with the specified index does not exist, Bookmarks[Index] returns -1, otherwise Bookmarks[Index] returns position of the bookmark. 

Writing: 

If bookmark exists, writing to Bookmarks property will change position of the bookmark. If assigned value is -1 and bookmark exists, this bookmark will be destroyed. 

If no bookmark has been found and assigned value is not -1, new TBookmark object will be created with the specified index and position. After creating bookmark OnSetBookmark event occurs, in which you can adjust properties of the new bookmark object.

TCustomSyntaxMemo Properties