Problem scenario
You have seen the term "base branch" in Git documentation. How can it not be the main branch?
Solution
In the context of a Pull Request, where one branch's changes will update the same files (if any exist) and any new files will be copied into another branch, the branch receiving the changes and/or files is the base branch. The term "base" is used in GitHub's GUI and Atlassian BitBucket.
For more information, see https://github.community/t/relationship-of-base-and-master/1715 or https://confluence.atlassian.com/bitbucketserver/using-branches-in-bitbucket-server-776639968.html