A CA system is simply one which is not available at all during a network partition, since it is partition-intolerant.

This lack of availability is different from the availability in the A of CAP, since that availability holds only so long as the network is not partitioned (by definition in a CA system).

Such a system might not be considered a distributed system at all (although it may still be distributing load), since a partition-intolerant system is effectively one system as far as the CAP theorem is concerned.

So it's essentially a special case of the CAP theorem, but it is still useful to describe it as CA.