I found that Internet Protocol (IP) version numbers are published by IANA. It refers to the following table.
https://www.iana.org/assignments/version-numbers/version-numbers.xhtml
| Decimal | Keyword | Version | Reference |
|---|---|---|---|
| 0-1 | Reserved | Jon_Postel, RFC4928 | |
| 2-3 | Unassigned | Jon_Postel | |
| 4 | IP | Internet Protocol | RFC791, Jon_Postel |
| 5 | Reserved (Historic) | RFC1819, SC589H | |
| 6 | IPv6 | Internet Protocol version 6 | RFC8200 |
| 7 | Reserved (Historic) | RFC1475, RFC6814 | |
| 8 | Reserved (Historic) | RFC1621, SC589H | |
| 9 | Reserved (Historic) | RFC1347, SC589H | |
| 10-14 | Unassigned | Jon_Postel | |
| 15 | Reserved | Jon_Postel |
Looking carefully, I found that IP version 15 was reserved by Jon Postel. I already know the uses of the other versions, but I have never seen or heard of IP Version 15. Why is IPv15 reserved? What is its purpose?