Hi there! The main difference between semi-para, para and novella(which is a less common writing style) is the length of your writing! While there is no set word count of sorts, typically, a semi-para would only have maybe 3-4 lines/sentences with less detail when compared to para which might involve enough writing to form a couple of sizable paragraphs. Novella goes beyond that: sometimes spanning over more than 5 paragraphs for each reply.

I've sent you a friend request in case you had any other questions you wanted to address! I hope I explained this well enough though!