Riffing on Jack's good info:

By dimensioning hole and features tolerances to true position rather than just plus minus, theoretically that true position is the same for drilling and photoplotting.

How do you know?

When your annular ring breakouts pass or fail.

So in the real world, you specify origin to a theoretical point in the universe and know if it works if the boards don't fail.  Don't worry, those fab guys and gals are magicians.  Well, magicians that use an artwork target as the drill origin.  That theoretical point turns out to be pretty accurate.  

Defining drill and artwork tolerances as Diameter True Position will determine which fabricators you can use.  When you define your hole to pad size ratio (and NPTH to copper clearances), you need to know what real world tolerance your fabricator can handle, and make the ring big enough to keep the holes inside.  

Pete

---------------------------------------------------------------------------------
DesignerCouncil Mail List provided as a free service by IPC using LISTSERV 16.0.
To unsubscribe, send a message to [log in to unmask] with following text in
the BODY (NOT the subject field): SIGNOFF DesignerCouncil.
To temporarily stop/(restart) delivery of DesignerCouncil send: SET DesignerCouncil NOMAIL/(MAIL)
For additional information, or contact Keach Sasamori at [log in to unmask] or 847-615-7100 ext.2815
---------------------------------------------------------------------------------