This may seem obvious but maybe not. I have an XS-32 and have used it for years but something may escape me.

Can someone explain what does the dvd compatible mode setting actually does? The manual says it is only effective for recording to a dvd-R/RW. For dubbing it does not matter.

I got a second XS-32 to replace my original and left the dvd compatible setting on "off" by mistake as it is the default. I then dubbed the show to a DVD-R and it played just fine.

Before I set it to "main" and it also worked fine after dubbing.

The manual is in Japanese English so I am confused.

What setting should I have it as if I am going to occasionally dub to a dvd? I assume main?

Can someone also explain what the SAP channel is and when it is used in conjunction to my main question?

SAP is second audio program. Some TV channels have this SAP, often a description of what is on the screen, or something completely unrelated to the video. Some machines will let you record the main audio on one channel, and the SAP on the other.