Changeset ID
A ChangeSet ID is a 160-bit identifier that uniquely describes a ChangeSet and its position in the change history, regardless of which machine it's on. This is represented to the user as a 40 digit hexadecimal number.
Short-form Changeset !IDs
Since a 40 digit hex number is unwieldy, ["Mercurial"] will accept any unambiguous prefix of a ChangeSet ID as shorthand for that ID.
When printing a ChangeSet ID, Mercurial usually prints just the first twelve characters. This is called the "short form" of the ID.
Note: a short-form ID can start out valid, but become invalid because another ChangeSet with the same prefix shows up later on. Do not use short-form IDs for long-lived representations.
Revision numbers are shorthand
Within a single repository, you can use a RevisionNumber as shorthand for a changeset ID.