The difference between Official and Unofficial builds
Each ROM team has its own requirements to approve a dev/builder as its official maintainer for a specific device. Some teams have stricter requirements (e.g. LineageOS) compared to others. If all requirements are met, then the ROM can be released with an official tag. If the requirements are not met and the dev/builder of a ROM hasn't been approved by the ROM team as an official maintainer or if they haven't applied yet for official maintainership, then their builds are considered unofficial.