values of column source in pg_settings

Hi all,

I could find several distinct values for column "source" in pg_settings
view. Examples of these values are: client, environment variable,
configuration file, default, database, override, session.

Are there other values?

The documentation doesn't mention any thing (Any way I didn't find).

In some of my databases, I have this:

postgres# select name, source from pg_settings where source like 'environ%';

name | source

-----------------+----------------------

max_stack_depth | environment variable

What is the meaning of 'environment_variable' here?

For the other listed values, I have "almost" got it but I don't see anything
for "environment variable".

Thanks in advance

Michel SALAIS