Basically this is a feature to glue a MIDI recording with a performance so the recording sounds as it was recorded instead of playing MIDI notes using a different performance.

From the reference manual:

File Type - Song&Perf:
A specified Song in a file that is saved to the dedicated
area in the User Memory as a “Song” type can be
individually selected and loaded (played back) to the
instrument. Both of MIDI sequence and Performance data
can be loaded.

Song&Perf has been an option since early firmware - later firmware fixed some problems with the initial code.

Although the firmware new-features docs are not about Song&Perf - the page you referenced does show the "STORE" touchscreen button for Song&Perf files.

imgh.us shutting down - backup file:

The same docs in page 18 show the load button, again not specifically documenting this feature. But the screenshot of the docs can be used to explain.

imgh.us shutting down - backup file:

This link contains a tutorial including use of Song&Perf:

https://www.yamahasynth.com/blog/montage-connect-ii

This is a bit less formal, but also describing how to use the feature:

https://www.yamahasynth.com/forum/how-to-link-a-performance-to-a-song

Even less formal - user-to-user help

https://www.yamahasynth.com/forum/attaching-a-song-to-a-performance

This discussion of how the feature was previously broken - and subsequently fixed may help:

https://www.yamahasynth.com/forum/song-performance-not-working

This tutorial mentions the feature without much more than a mention:

https://www.yamahasynth.com/resources/performance-basics-and-the-live-set-1

Hope that helps.

Found this information typing:

site:yamahasynth.com store song&perf montage

... as the search term in a search engine that starts with "G". And a PDF search in the owner/reference manuals.

What does it do specifically? I can take a guess based on its name, but I'd rather find out the actual facts from someone who knows.

It places a marker with your Song data that links it with a Performance so they can be recalled together when you Load the Song title.