_MST2_ TTL is the time to live of the records. You usually don't want it too short, nor too long in case you need to do an update.
I suggest you keep whatever defaults your DNS provider use, or do some research and decide for yourself what value is the best.
It would be best to get a response from @Sanguine , but it shouldn't matter for FreeFlarum because he's using CNAMEs, so he controls his own TTL on the CNAME record and can update it accordingly.
I'm not sure what prio is. Priority maybe ? Not sure if that's part of the DNS spec for CNAMEs...