@Sven *******
I believe you're confusing HOW you show that income,
IF you come from a country that doesn't issue the income affidavit
AND
don't legally work inside thailand on a work permit which shows 40K baht a month in salary, that means you have to transfer the funds in 40k baht a month, each month, every month for the previous 12 months before you apply for a yearly extension.
This means you have to use banked money the first year and can only use monthly income by transfer method the second year