mercurial/help/revisions.txt

Mercurial supports several ways to specify individual revisions.

A plain integer is treated as a revision number. Negative integers are
treated as sequential offsets from the tip, with -1 denoting the tip,
-2 denoting the revision prior to the tip, and so forth.

A 40-digit hexadecimal string is treated as a unique revision
identifier.

A hexadecimal string less than 40 characters long is treated as a
unique revision identifier and is referred to as a short-form
identifier. A short-form identifier is only valid if it is the prefix
of exactly one full-length identifier.

Any other string is treated as a tag or branch name. A tag name is a
symbolic name associated with a revision identifier. A branch name
denotes the tipmost revision of that branch. Tag and branch names must
not contain the ":" character.

The reserved name "tip" is a special tag that always identifies the
most recent revision.

The reserved name "null" indicates the null revision. This is the
revision of an empty repository, and the parent of revision 0.

The reserved name "." indicates the working directory parent. If no
working directory is checked out, it is equivalent to null. If an
uncommitted merge is in progress, "." is the revision of the first
parent.