SQLite Forum

SQL compliance on Wikipedia
from the page cited:

> The second syntactical form, <b>called a positioned update</b>, updates one or more columns on the current row of an open, updatable cursor. If columns were specified in the FOR UPDATE clause of the SELECT statement used to generate the cursor, only those columns can be updated. If no columns were specified or the select statement did not include a FOR UPDATE clause, all columns may be updated.

There ain't no mystery in that. E121-06 is just what is called a "feature ID":

> Positioned UPDATE statement — Subclause 14.9, ‘‘<update statement: positioned>’’: Without support of either Feature E153, ‘‘Updateable tables with subqueries’’ or Feature F221, ‘‘Explicit defaults’’

(from https://web.cecs.pdx.edu/~len/sql1999.pdf; I know it's 1999, but should be enough for answering the queastion)