Two different things, although related. Each breaker must be able to safely interrupt the expected short circuit current through it.
As a way to avoid buying more expensive breakers with high interrupting ratings, it is possible to rely on a upstream breaker, normally larger, to "help" interrupt the high magnitude faults IF THESE BREAKERS HAVE BEEN TESTED IN COMBINATION and are UL listed for "Series Rating".
Selective coordination refers to the concept that in case of a fault, the breaker closest to the fault should trip to interrupt the fault, thereby limiting the outage to the smallest portion of the system as possible.
If as system is designed to rely on series rating for safely interrupting a fault, it is impossible to have a fully selective system, since the upstream breaker must open to protect the downstream breaker.
So basically, if you can save money by relying on series rating, but you give up any notion of selective coordination if you do so.
There is a great fallacy being circulated, particularly by certain fuse manufacturers, to the tune of you can choose selective coordination or you can choose series ratings as though they were mutually exclusive. It is true that with a series rating that you won't have selective coordination, but the converse is absolutely not true. The real choice is between series rated and fully rated. Most fully rated combinations, where each device is rated to interrupt the available fault current without assistance, are not selectively coordinated.
Fuses, when selected per certain tables, will provide selective coordination, but then you have all of the disadvantages of fuses.
With circuit breakers, full selective coordination is difficult to achieve when using Thermal-Magnetic trip units. In many cases the trip curves of breakers that would form a series rated combination are identical to the trip curves of breaker that would form a fully rated combination. You can't tell a fully rated combination from a series rated combination by trip performance alone.
I agree that, in most cases, any molded case circuit breakers in series will not fully coordinate, due to the instantaneous trip units that are required. No question about that. It's not like there is anything unique about MCCBs that have a series rating other than the fact that they have been tested as a combination. The trip characterisitics are the same.
If low voltage power circuit breakers are used and properly applied, full selectivity can generally be achieved.
Sometimes (not always) current limiting circuit breakers are used upstream to obtain series ratings. The current limiting behavior is recognizable on the instantaneous portion of the time-current curve by a decrease in clearing time as current increases.
