Firebird is a multi-generational database. When a record is updated, a copy of the new values is placed in the database, but the old values remain (usually as a bytewise difference from the new value). The old value is called a "Back Version". The back version is the rollback log - if the transaction that updated the record rolls back, the old version is right there, ready to resume its old place. The back version is also the shadow that provides repeatable reads for long running transactions. The version numbers define which record versions particular tranasctions can see.
When the transaction that updated the record commits and all concurrent transactions finish, the back version is unnecessary. In a database in which records are updated significantly and regularly, unnecessary back versions could eventually take up enough disk space that they would reduce the performance of the database. Thus they are GARBAGE, and should be cleaned out.
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