What is the difference between a "electronic preprint server" and a "subject repository"? In the following licence to publish: 

6.b) preprints (before referee) can be posted to electronic preprint servers

6.c) postprints (after referee) can be posted to subject repositories

Now I am wondering whether arxiv is an electronic preprint server (wiki says it is) a subject repository for math (wiki says it is) or neither or both.

As I could not find any useful distinction between these two terms online, it feels appropriate to ask the question here.
I do not know of any agreed-upon definition of these terms that would clearly distinguish one from the other.

It would be appropriate to ask the publisher to clarify.  You may wish to ask specifically about the sites where you would like to upload your work.