Gmail bak folder?

[Mathias Korber]

I started backing up a number of my Gmail/Google Apps accounts
uwing offlineimap and I find that for several of them (all?)
there seems to be a folder 'bak' (with subfolders)
that I don't see in Gmail itself

Is that the Archive?
Is that duplicating any emails (the numbers of messages in those
folders are huge for some accounts, 100k messages and counting)

Should I filter that or are those messages potentially useful?