Ctr
The regulation in question requires the filing of a Currency Transaction Report (CTR), whenever your cash transactions for the day total $10K or more. Most places start asking for identification when you get to $3000 or so, because if you make several transactions during the day you could hit the $10K important number.
This is NOT a form filed with the IRS claiming the $10K (or whatever amount) is income. Instead, it just means that you made a transaction totaling at least that amount in currency. The main intention is to prevent money laundering and to track large amounts of cash that can be an indicator of illegal activity.
These forms get filed with the Financial Crimes Enforcement Network (FinCEN). It's not a tax thing, though I'm sure that your other IRS tax forms might be cross-checked against these. For example, if you claim a low income and then transact hundreds of thousands in cash at the casino, you probably have a problem.
Intentionally 'structuring' your transactions to avoid the reporting requirements is a serious crime as well. This is one reason that some players hoard chips over time to avoid cash transactions that cause identification requirements and extra paperwork and attention from the casino.