The IB ratifed the following policy at the June 2020 B2GM:
Belle II strongly profits from, recognizes the value of, and supports open source software. By committing code to a Belle II repository the right to be recognized as original author is granted and the author allows the members of the collaboration to use and modify it.
A Belle II repository can be declared public if all contributors agree that it can be distributed under an open source license. By committing code to a public Belle II repository contributors agree to the publication of their code and affirm compliance with the code of conduct. The copyright remains with the contributors.
The considerations that led to the policy are documented in the internal note BELLE2-NOTE-TE-2019-028.