4-24/ ASME /2009

[Andera]

Learning ASME stuff and I got the following question:
4-24 / 2009

Is it legal to call A(MMB) in the perpendicularity callout?

If yes, then why A is RMB?
Otherwise stated, why datum A is MMB in the position and RMB in the perpendicularity?

 

[greenimi]

https://www.eng-tips.com/viewthread.cfm?qid=451136

See this discussion.

Q1: I don't think that there are any rules or limitations on the boundary condition(s) that datum features can be referenced at in relation to the material condition that the datum feature's tolerance has. A datum feature can be referenced RFS in one FCF and referenced at MMC in another FCF on the same drawing.

I am not sure myself, why standard promotes such of practice.
In my opinion it is just a very bad practice.