What is the correct information to enter in the 'Visa Number' section of the arrival card when using a re-entry permit in Thailand?

Okay, so now we know IF you are on a VISA <- meaning a sticker from a thai consulate from another country, you put that number on it.

BUT

IF you are on an extension of stay <- a STAMP from the immigration office INSIDE the country & have a re-entry permit <- also a stamp from the immigration office inside the country; Put the re-entry permit number on the "visa number" spot of the TM.6..

Example:

In this example image the extension of stay number is <- that is NOT the number you put on the TM.6 card

the re-entry permit number is <- that IS the number you put on the TM.6 card

If you have a visa you put the visa number. If you have a reentry permit, you put the reentry permit number.

On my four return trips to Bangkok I have always entered my Visa Number in this space and have never been pulled up. After what Geoff passed on, should this space be left blank, even though the heading is “Visa Number”?

So you made a mistake, the IO is informing this to you and correct the mistake and you call this a minor point? I hope minor point of yourself in stead of the correction made by Immigration.