What IR model the PostgreSQL FTS implementation is based on?

Hi,

I'm trying to figure out what information retrieval model[1]
the PostgreSQL Full-Text Search is based on.

Based on my reading on the PostgreSQL documentation I would have guessed
extended boolean model but I came across an web article by Compose[2]
which claims that FTS is based on vector space model instead. The
article does not offer anything to back this claim so I thought to ask
here if anyone has more accurate details to share.

I've tried to look this up from postgresql sources but so far I have not found
anything.

Timo

1: https://en.wikipedia.org/wiki/Information_retrieval#/media/File:Information-Retrieval-Models.png
2: https://www.compose.com/articles/indexing-for-full-text-search-in-postgresql/