What happens after the Thai 90-day report expires?

The question relates to the 90-day reporting requirement for foreigners in Thailand, but many commenters expressed confusion over what exactly is being asked. The 90-day report can be done within the last 15 days of the reporting period and grants a 7-day grace period after it expires. Beyond this grace period, fines may apply. Clarification on which specific 90-day period is necessary to provide an accurate answer.