The issuance of multiple types on a single day is an old practice, but usually these were different denominations, and could be numbered in ascending order of value.
Logically, they are part of a single group, with a unified design theme and a sequence of values, even though ten years or more may have elapsed from the first to the last.
Because of its commercial importance the publishers of the Scott Catalogue claim copyright on their numbering systems, and grant only limited licences for their use by others.
The inconsistency with which Scott enforced these licences resulted in a lawsuit by Krause Publications (publishers of the Minkus Catalogue) for copyright infringement.
In 2002, as part of efforts to control allegedly illegal stamps, the Universal Postal Union introduced the WADP Numbering System (WNS) for new issues by UPU members.