Why Patek made its own
Pre-2009, Patek Philippe applied for Geneva Seal certification on most of its watches; the Seal was a key brand-prestige marker. Patek leadership (Thierry Stern) decided that Geneva Seal had become insufficiently strict for Patek's positioning, particularly because the Seal didn't test the watch with a complete bracelet/movement-in-case configuration to Patek's accuracy expectations. In 2009 Patek announced its own internal seal applying to cased watches, with stricter accuracy and explicit finishing requirements.
What it tests
Accuracy in case: -3 to +2 sec/day across multiple positions. Stricter than COSC (-4/+6 movement-only) and stricter than Geneva Seal (-1/+10 cased). Movement finishing: full anglage, polished sinks, hand-engraved balance cock on Patek hand-finished references, polished and chamfered jewel countersinks. Case and dial: Patek's internal quality criteria for diamond-set, enamel, and metal-case finishing. Bracelet: similar internal criteria. The seal is applied only to watches passing all four criteria.
How it compares
Patek Seal vs COSC: Patek is roughly 2x stricter on rate and tests the cased watch (COSC tests bare movement). Patek Seal vs Geneva Seal: Patek is stricter on rate (-3/+2 vs -1/+10) and includes case/dial/bracelet criteria the Geneva Seal doesn't address. Patek Seal vs Master Chronometer: Patek is stricter on rate; Master Chronometer adds 15,000-gauss anti-magnetism (Patek doesn't test for this).
What it means for a buyer
Every modern Patek (post-2009) production wristwatch carries the Patek Seal. The certification doesn't show on the dial; it's stamped internally and verified through the watch's papers/warranty. As a buyer, the Patek Seal is genuine quality assurance for a CHF 25,000+ watch, but it's not differentiating within the Patek catalogue, since every Patek has it. Other haute-horlogerie brands have similar internal standards; the seal is more about brand control than buyer signalling. See wiki: Patek Seal.