A Work is the logical concept of an Act or other piece of legislation. A Work may change over time through amendments or be issued in multiple languages.
An Expression is a particular version of a work, in a specific language and at a particular point in time. A Work may have many expressions associated with it.
An Amending Work is an Act or other legislation that makes changes to another work (the Principal Work). These are published when government needs to update or change an existing piece of legislation.
Amendments are the changes that are applied to a Work by an Amending Work. They are instructions of insertions and deletions that must be made to the text of the Work being amended. Amendments are similar in concept to the diffs used by programmers that are produced by tools like git.
A Point in Time is all the Expressions of a Work on a particular date. There is usually only one Expression at a particular date, unless the Work is available in multiple languages. Often, a Point in Time is synonymous with an Expression.
Some Acts have associated subsidiary legislation, such as regulations or other types of statutory instruments, that are related to the primary Act.
The terms Work and Expression are from the Functional Requirements for Bibliographic Records (FRBR) model.