"innovative Serializable Snapshot Isolation (SSI) level"

The following documentation comment has been logged on the website:

Page: https://www.postgresql.org/docs/12/mvcc-intro.html
Description:

The documentation page:
__https://www.postgresql.org/docs/12/mvcc-intro.html
makes a reference to the term "innovative Serializable Snapshot Isolation
(SSI) level"
I can't find that anywhere in the documentation. I am expecting to find it
here:
__https://www.postgresql.org/docs/12/transaction-iso.html
or here:
__https://www.postgresql.org/docs/current/sql-set-transaction.html
But there is nothing.

A search for the term "innovative Serializable Snapshot Isolation (SSI)
level":
__https://www.postgresql.org/search/?q=Serializable+Snapshot+Isolation+
produces a lengthy listing, demonstrating that the term is used throughout
the documentation. But there is no entry in the documentation that defines
the term, or that defines the commands required to use it.

It is important to know:
a. if this is an ANSI/ISO/SQL Transaction Isolation Level
b. or if it is an extension
c. or if it is neither
d. how to use it