[ape]

Pinie,

The OmniGraffle online help has a section on this. I've included most of it below:

Insert Variable
While you are editing a shape object's text, select one of these commands to insert a special tag. When you finish editing the text, the tag changes to represent some aspect of the object itself, the canvas, the page, or the document. If you change the thing the tag refers to, the tag automatically updates itself.
If you are using one of the date variables, you can customize the format of the date using Cocoa's date formatting tokens. For example, to get a date like "2005 - 10 - 06 16:00", you could type this:
<%date %Y - %m - %d %H:%M %>
Canvas Name — the name of the canvas the object inhabits.
Document Name — the name of the document the object inhabits.
Current Date — the date right this very minute; this is nice for knowing when a document was printed.
Document Creation Date — the date that the document was originally created, according to Mac OS X's file system.
Document Modification Date — the date that the document was last saved, according to Mac OS X's file system.
Creator — the name of the user who created the file, according to Mac OS X's file system.
Modifier — the name of the user who last saved the file, according to Mac OS X's file system.
Page Number — the number of the page that the object inhabits, according to the Canvas: Size inspector and the Page Setup panel.
Total Pages — the number of pages in the whole document. Use this with the Page Number tag to get results like "Page 2 of 5".
X Position — the object's horizontal distance from the origin, just like in the Properties: Geometry inspector.
Y Position — the object's vertical distance from the origin, just like in the Properties: Geometry inspector.
Width — the object's horizontal size, just like in the Properties: Geometry inspector.
Height — the object's vertical size, just like in the Properties: Geometry inspector.
Line Length — This tag is somewhat unique. When the shape object containing the tag is attached to a line as a label, it shows the line's length. It's not useful if the shape object is not a line label.