The word "subscribe" can have different meanings. Here, I believe it means "contribute" or "accept/permit". So, "Maximum subscribed flow" is the maximum permitted flow. "Average subscribed flow" could be the average flow the computer has contributed with or accepted/permitted.

Mind you that the word "subscribe" is ambiguous. I have seen it in use as a synonym to "request". I mean, that the data requested is the data the device has subscribed to. But I don't think that's the case here, I'm just telling you that the word could be somewhat confusing sometimes.