Shafts are smaller than the nominal size.
Standard CNC milling, turning, and general machine work. IT12 to IT18: Rough machining, drilling, and raw castings. Fundamental Deviations (Letters) iso 286 pdf
ISO 286 is used daily in:
The primary purpose of ISO 286 is to define the —the relationship between the hole and the shaft. Shafts are smaller than the nominal size
While summary tables are available online, the official offers specific advantages: the official offers specific advantages: