Each individual commit againt Subversion is considered one revision. The number reported by Subversion is just the sequence number of this commit. For example, if there is a jump of 94 between any two of your commits, one or more other users of the subversion repository have completed 93 other commits.
In some development environments, in addition to people, there are also automated processes, for example. Automated assemblies that create tags or branches, and therefore increase the number of versions.
The algorithm used by Subversion is very simple: start from scratch when creating the repository, then count the number of successful commits against the repository. Note that commits are atomic (or transactional) and therefore fully sequential, similar to a transactional database system.
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