Importing Apple mail archives

Hi -

I have a client who recently had their Apple mail app crash and i needed to update to High Sierra to sort it because the mail folders had got too large to make it worthwhile to do a Time Machine restore.
The client now has all email accounts back but the messages only go back to last week (despite being on IMAP i dont think it was set to save much on the server).
They have asked if i can restore the emails from 2017 from some of the archives that were made at the end of 2017.
I did not make the archives so i had to search for them. I found a folder with the heading archives 2017 but the folders are several, totaling to 22GB but the one folder called archive.mbox is only 2 GB. The largest one i could find has a long name which does not initially make any sense. I have attached a screenshot.
I have initiated an  import of the folder but being being so large i will have to wait a while before i know if it is the correct one. Could someone explain to me why these folders are created and named in such a way if the secretary who did the archive i assume followed the correct procedure. Why isn't there just one MBOX file?

Thanks
D
Apple-mail.PNG