Origination of "For" tag in Illustrator PDF saved with "Preserve Illustrator Editing Capabilities"?

The %%For is a commented section in the header of a Postscript/EPS file that identifies the account user name followed (sometimes) by the computer's name. This is one of a bunch dictated by the Postscript Language Document Structuring Conventions (DCS) Specifications. Most programs capable of saving PS/EPS files and PS print streams will have this information if it exists.

This information is less relevant these days but was helpful/necessary in print workflows to identify the file's source in a print queue. It usually follows the %%Creator and %%CreatorVersion. like so:

So to answer "Where does Illustrator get the information from to write this tag?"

It's provided by your System to Illustrator

"Can I retrieve this data using Illustrator ExtendScript?"

Couldn't tell you, but like @Ton Frederiks mentions, maybe someone with scripting experience will chime in.

"If I wanted to read/write to this location, what would be the namespace where it would be found?"

In an exported PS/EPS files, it's in the header at the beginning of the file in plain text. In a PDF saved with Illustrator Editing enabled, it's part of the .AI/PS code attached so it can also be found by a text editor somewhere in the file.

As far as I can remember Illustrator has always written the name of the registered Illustrator user in the file.