Microsoft uses the CARO naming scheme for malware identification. The suffix part (name's right-most part after the ! separator) is supposed to be dedicated to "vendor-specific" comments [1]. Microsoft does list a few suffixes [2], but not MTB.
Does anyone knows what this suffix means ? It maybe maps to an "internal category" like others suffixes (eg. : !bit ), but it isn't listed, so I'm not sure.
This suffix seems to come with many types of malware (leftmost part of name), from html to trojan.
[1] https://web.archive.org/web/20141107015518/http://www.caro.org/naming/vendorcomment.html
[2] https://learn.microsoft.com/en-us/microsoft-365/security/intelligence/malware-naming
