Insider Buying vs Insider Selling: What Form 4 Codes Mean

Not all insider transactions are created equal. A headline that says "the CEO sold $4 million of stock" can mean I'm worried about the company — or it can mean my restricted shares vested and the company automatically withheld some to cover my taxes. The only way to tell the two apart is to read the transaction code on the SEC filing. Once you learn the handful of codes that actually matter, insider data goes from noise to signal.

Where the codes come from

Insiders — officers, directors, and 10%+ owners — report trades on Form 4, filed with the SEC (and visible on EDGAR) generally within two business days. Every line on a Form 4 carries a one-letter transaction code in a column literally labeled "Code," defined by SEC rules. The code tells you how the shares changed hands, which is what separates a conviction buy from a compensation event.

The codes that signal conviction

P — Open-market or private purchase. The most important code for signal-hunters. The insider chose to buy shares with their own money at the prevailing price. Nobody made them do it. This is the buy that research links to outperformance, especially when it clusters across several insiders.

S — Open-market or private sale. The mirror image: a discretionary sale. This is the code that can be a warning — but read it carefully, because selling has many innocent explanations (taxes, diversification, a major purchase). One sale is rarely a signal; a wave of large, discretionary sales at the same time is more interesting.

The codes that are mostly noise

Most Form 4 lines are not discretionary bets at all. They are the mechanics of how executives get paid:

• A — Grant or award. Shares or units handed to the insider as compensation. They didn't buy anything; the board granted it. Not a market signal.
• M — Exercise of derivative (options/RSUs converting to shares). The insider converted previously granted options or units into common stock. Routine.
• F — Shares withheld to pay taxes. When equity vests, the company often withholds shares to cover the tax bill. This shows up as a disposition but reflects zero opinion about the stock — it is bookkeeping.
• G — Bona fide gift. Shares given away (e.g., to family or charity). Not an investment decision.
• C — Conversion of a derivative security. A mechanical conversion. Routine.

The classic mistake is seeing a big "sale" number and panicking, when the underlying codes are M (option exercise) followed by F or S to cover taxes — a pre-planned, mechanical event with no informational content. The opposite mistake is treating an A grant as if the insider "bought" stock. They didn't; it was given to them.

Planned vs. discretionary: the 10b5-1 footnote

Even a code-S sale can be non-informative if it was made under a Rule 10b5-1 trading plan — a pre-arranged schedule the insider set up months earlier, specifically so they can sell without being accused of trading on inside information. Form 4 now flags whether a transaction was made under such a plan. A scheduled, automatic sale tells you far less than an unplanned one. The same logic applies in reverse: a discretionary, off-plan purchase is the strongest version of a buy.

Why buying is the cleaner signal

This is the core asymmetry, and it is worth repeating: insiders sell for many reasons, but they generally buy for only one — they expect the price to rise. Compensation, taxes, gifts, diversification, and liquidity all push insiders to dispose of shares. Almost nothing pushes them to spend their own after-tax cash buying more, except a belief that the shares are undervalued. That is why a screen built for signal focuses on code P, open-market purchases — and why this site tracks purchases specifically.

Putting it together

When you read a Form 4, do this:

1. Find the code. P and S are discretionary; A, M, F, G, C are mostly mechanical.
2. Check for a 10b5-1 flag. Planned trades carry less information.
3. Size it. Dollars committed and change in total holdings matter more than the headline.
4. Look for clustering. Several independent code-P buyers beat one of anything.

Ready to apply it? Scan current open-market purchases on the cluster-buy screener, then open a dashboard like TSLA or META to see the filings in context with price and sentiment.