Rate tolerance is the interval within which a movement is considered acceptable: not a measure of absolute perfection, but a technical contract between maker and buyer. It is expressed in seconds per day and always carries two signs — positive for gain, negative for loss — or only positive when the maker deliberately chooses to exclude loss, as Hanhart does with the Soprod P024 (0/+8 s/d) to guarantee the watch never runs slow.
The historical reference standard is COSC, which certifies dismantled movements for 15 consecutive days according to ISO 3159: the allowed tolerance is -4/+6 seconds per day. It is a robust standard, but it measures the caliber naked in the laboratory, not the watch you wear. METAS, the Swiss Federal Institute of Metrology, changed the rules in 2015: it certifies the movement already mounted in the case, imposes tighter tolerance (0/+5 s/d), verifies magnetic resistance up to 15,000 gauss and checks declared waterproofness. A movement can pass COSC and fail METAS: they are not equivalent. METAS even audits COSC's internal processes.
Rolex with Superlative Chronometer certification brings tolerance to -2/+2 s/d, measured on the complete watch. Patek Philippe Seal reaches -3/+2 s/d on the finished caliber. The COSC Excellence Chronometer, an updated version of the classic standard, provides ±2/+4 s/d with magnetic tests at 200 gauss and five additional days on a wearer robot that simulates real wear. Different numbers, different methodologies: comparing them without contextualizing the measurement method is an error I see often even in specialized publications.
On the bench, declared tolerance is only the starting point. An ETA 7001 movement in Top Grade finishing leaves the factory regulated within chronometric standards, but real tolerance after six months of use depends on wear, lubrication and the watch's nighttime resting position. Grand Seiko's MEMS technology — which produces escapement wheels with tolerances of one millionth of a gram — reduces variation between pieces produced in series, but does not replace the final regulation of the regulator. A Miyota Caliber 82 movement with barrel rolled to 0.01 mm tolerance is an example of how mechanical production precision translates, downstream, into more stable rate tolerance over time. Declared tolerance on the technical sheet is a photograph at the moment of test: what happens after is true watchmaking.