Actually, Unstable is the development distribution. it’s just formal definition from debian build files. The sequence of package propagation in the Debian development process is as follows:
→ experimental→ unstable → testing → stable
We have 2 main repo: experimental and main. all packages targeted for release will be uploaded to experimental. if no bugs found - packages moved to main without rebuild. so, that’s the reason why package may have unstable record. all packages released to ‘main’ repository can be used for production so you can upgrade to it.
Just FYI: unstable means rather distribution than package stability. unstable distribution has packages which may be changed often. so - unstable. stable package is ‘freezed’ packages, without any changes for the years. https://wiki.debian.org/DebianUnstable