Every time a corporate insider (officer, director, or anyone owning more than 10% of a class of shares) trades stock in their company, they have to tell the SEC within two business days. The vehicle is a Form 4, and the transaction itself is summarized by a one-letter transaction code.
Understanding the codes is the difference between "big insider buy!" headlines that are actually option exercises and a real open-market purchase where the CFO put $500,000 of personal cash on the table.
The codes that matter
P: Purchase
The only code that represents a real open-market buy. The insider bought common stock on the open market with their own money, at the prevailing market price. P is the strongest single-transaction signal a Form 4 can carry. Insiders almost never buy for the fun of it. They buy because they expect the price to be higher later. Aggregate P activity across multiple insiders within a short window is worth attention.
S: Sale
An open-market sale of common stock. Ambiguous on its own: insiders sell for dozens of reasons unrelated to outlook (paying taxes on a vest, diversification, buying a house, divorce settlement). What matters is context:
- Is this part of a Rule 10b5-1 trading plan? If yes (check the footnote), the sale was scheduled months in advance. Not a reactive signal.
- Is this the insider's first sale in years? That's notable.
- Is the sale a large percentage of the insider's total holdings? Also notable.
M: Exercise of options
The insider exercised vested stock options, converting options into common stock. This is not an insider buy. The shares were effectively awarded via the original option grant; M is just the accounting moment they become common stock. Almost always paired with F (tax withholding) and often S (immediate sale of the converted shares).
F: Tax withholding
Shares withheld by the company to cover taxes on a vest or exercise. Pure mechanics: not a market transaction, not a signal.
A: Award/grant
Shares granted by the company, typically as part of equity compensation (RSUs, performance shares, stock bonuses). Not a buy in any meaningful sense; the insider didn't put up cash.
G: Gift
Shares given away or received as a gift. Common vehicles: charitable donations, family trust transfers, estate planning. Occasionally a bellwether of outlook (a big charitable donation can signal an insider thinks the price is high right now) but usually routine.
The codes you'll rarely see
- C: Conversion of derivative security. Warrants or convertible preferred turning into common stock. Mechanical.
- D: Disposition to issuer. Company bought the shares back directly from the insider, typically at a pre-set price. Planned.
- J: Other acquisition or disposition. Catchall. Footnote will explain.
- W: Expiration of option. An option expired worthless or unexercised. Mild negative signal (insider didn't think the stock would clear the strike price before expiration).
- Z: Trust transfer. Shares moved between trusts the insider controls. Mechanical.
- I: Discretionary transaction. Broker-executed on the insider's behalf, typically to satisfy tax obligations.
- K: Equity swap. Rare derivative restructuring. Read the footnote.
- U: Tender of shares. Shares tendered in a takeover or buyback offer. Event-driven.
What to actually do with a Form 4
- Filter for P. Ignore everything else on a first pass. P tells you an insider voluntarily bought.
- Check the size. $10K purchase from a director with a $50M net worth is different from $500K purchase from the same director. Both are signals, but one is a much larger personal bet.
- Look for clusters. Two insiders buying in the same week is interesting. Five insiders buying in a month is a pattern worth investigating.
- Read the footnote. Almost every Form 4 has a footnote explaining the economic context. Especially on S transactions, the footnote tells you if it was a scheduled 10b5-1 sale or a reactive sale.
- Cross-reference with 8-Ks and earnings. Insider selling right before an earnings miss is different from insider selling into a news vacuum.
How Portolio surfaces Form 4s
Every Form 4 on Portolio shows the transaction codes as colored pills (P in green, S in red, M in violet, F in amber) next to the insider's name and title. You can see at a glance whether the filing is a real purchase, a compensation event, or a scheduled sale. The /discover/insider-buys page filters directly to Form 4s with transaction code P (the high-signal subset), updated as each one lands on EDGAR.