The suffix is the text that comes after the number in automatic naming. For example, in AD01 - Lip Filler, the suffix is Lip Filler. With Keep current suffix turned on (the default), new ads follow the ad set's pattern: AD06 - Lip Filler. With it turned off, only the number is used: AD06. The suffix is read from the first ad already in the ad set, so each ad set can have a different suffix.