Files with bso file extension might be found as so called BSML overlay files.
BSO file extension - BSML overlay
What is bso file? How to open bso files?
File type specification:
The bso file extension is mainly used for BSML overlay files.
An overlay file consists of BSML fragments, which can be overlaid onto an existing BSML document. For example, two researchers can swap overlay files for a base sequence record, and more easily exchange and compare sequence annotations.