Last leg
I think now that Dave is back on subject the answer lies in his last reply. In Series rating there is NO coordination except accidentally. The right breaker might ultimately trip alone but it is completely possible and part of the design that both the branch and main breakers will trip.
If you want coordinated tripping there are some simpler methods and complex methods to achieve coordination. For me coordination means the O/C device closest to the fault is the only one that trips. It is easiest to achieve in fuses and electronic controlled breakers. Thermal magnetic breakers can be coordinated but as Dave said it is a little less reliable than a more sophisticated electronic trip control breaker.
If the load is really critical don't go for series rated gear.
Coordination is a very complex subject and is an absolute necessity in some factories never mind hospitals data centers, etc.
