I am trying to read a CAD drawing of a tool and there are two places where -A- and -B- are written, I'm not sure what they mean.  I was hoping someone here could tell me.  Thank you for your help.Nici

 
 Maybe this can you help.
 
 Datum - A theoretically exact plane, point or axis from which a dimensional  measurement is made. 




Datum Feature - A part feature that contacts a datum.