Two filings can both show an insider ending the day with more shares, and mean very different things. One is an open-market purchase, transaction code P, where the insider pays the going market price with their own money. The other is an exercise of options or other derivatives, code M, where shares arrive by converting a right the company granted earlier. Screening tools sometimes lump them together as insider buying. Readers should not.
A code P transaction is a purchase in the open market or a private purchase reported as such. The insider instructs a broker, pays the market price, and takes the same risk any other buyer takes at that moment. There is no discount built into the transaction and no compensation element. If the stock falls the next week, the insider loses money on exactly the same terms as every other shareholder who bought that day. That symmetry is what gives the code P buy its reputation as the cleanest insider signal: it is voluntary, it costs cash, and it increases exposure the insider already has through pay.
Details strengthen or soften the reading. A purchase made under no plan footnote is a live decision. The size relative to the insider salary and existing holdings tells you how much conviction the dollars represent. And under Section 16(b), the buyer cannot sell at a profit within six months without returning that profit to the company, so a code P buy is a commitment measured in months, not days.
Code M covers the exercise or conversion of a derivative security, most commonly employee stock options. Options are compensation. They were granted in the past, often years earlier, with an exercise price set at the market price on the grant date. When the insider exercises, they pay that older exercise price, which may be far below the current market price, and receive shares. The economic gain was largely created by the grant and the stock movement since, not by a new decision to invest at today prices.
Exercises are frequently driven by deadlines rather than views. Options expire, often after ten years, and unexercised in-the-money options become worthless at expiry or shortly after an executive leaves a company. Exercising near expiry is housekeeping. Many exercises are also exercise-and-sell transactions: the insider exercises and immediately sells the shares, sometimes in a cashless exercise arranged through a broker, ending with no increase in exposure at all. The Form 4 will show the code M acquisition and code S sales on the same filing, a pattern that should never be counted as buying.
Three checks separate the two stories. First, the transaction code itself: P is a purchase, M is a conversion. Second, the price column: an exercise price well below the recent market price confirms you are looking at an old grant, not a market decision. Third, what happens next: if the insider holds the exercised shares, exposure genuinely rose, and that is mildly positive context. If the shares are sold the same day, or withheld under code F to cover taxes and the exercise cost, the net position may be flat or down.
Holding after exercise deserves a fair note. An insider who exercises and holds is choosing to keep risk they could have cashed out, and they often owe taxes on the exercise gain. That is a real decision. It is still weaker evidence than a code P purchase, because the entry price was set in the past and part of the choice is tax and expiry management.
The simplest discipline is to build your screens on code P only, then read code M filings individually when they appear at companies you follow. That ordering keeps compensation mechanics out of your buy list and reserves your attention for the transactions where an insider reached for their own cash.
Nothing here is investment advice; it is a guide to reading transaction codes accurately before drawing any conclusion of your own.