13D filing (>5% ownership)

A 13D filing is a public document an investor must submit to the SEC when they buy more than 5% of a company's stock. Think of it as a transparency rule—once you own that much, the market needs to know about it. You'll encounter these filings when tracking major ownership changes or potential takeovers, since someone buying 5%+ often signals serious intentions about influencing the company. The filing includes who the buyer is, how much they paid, and what they plan to do with their stake. For example, if an investment firm quietly accumulated 6% of a retailer's shares, they'd have to file a 13D disclosing their position and whether they're pushing for board seats or a merger. It's basically the SEC's way of keeping big moves transparent.