Is the other dowel referenced in the dual specification to |A|M-N| and |A| by (a "2X" preceeding the size) or by (two leader lines extending from the FCFs to both holes) or by (a note including both hole under the FCF) or by (some other non-standard method)?
If not then the bottom note is only a refinement of perpendicularity of diameter "B" to surface A as others have stated.
If the size is preceeded by 2X and the datum feature identifier "B" is underneath the FCF or on the leader line to that FCF then both dowels simultaneously are Datum feature "B" and all six degrees of freedom would be constrained for related features by designating just |A|B|.
If the designations "B" and "C" are applied individually to each dowel hole away from the FCF and the two holes are controlled as a pattern as questioned above then they are unique datum features. If other feature control frames reference them as |A|B|C| they are secondary and tertiary respectively but their spread is still controlled by the bottom note relative to A
It is difficult to comment without a drawing especially if it is non-standard and I don't have AutoCad. You would have to provide a sketch with all the relevant designations, leaderlines, identifiers, etc...
If not then the bottom note is only a refinement of perpendicularity of diameter "B" to surface A as others have stated.
If the size is preceeded by 2X and the datum feature identifier "B" is underneath the FCF or on the leader line to that FCF then both dowels simultaneously are Datum feature "B" and all six degrees of freedom would be constrained for related features by designating just |A|B|.
If the designations "B" and "C" are applied individually to each dowel hole away from the FCF and the two holes are controlled as a pattern as questioned above then they are unique datum features. If other feature control frames reference them as |A|B|C| they are secondary and tertiary respectively but their spread is still controlled by the bottom note relative to A
It is difficult to comment without a drawing especially if it is non-standard and I don't have AutoCad. You would have to provide a sketch with all the relevant designations, leaderlines, identifiers, etc...