The discipline of designing a board so it can actually be built reliably and cheaply.
How it looks
Real image: CORE3588, rendered by KiCad from our board file
U1 on CORE3588 (the eFuse chip, 2 x 2 mm). A DFM decision in real life: the chip's pads cannot be spread further apart, so the design checked the fab's table and wrote a rule just for this part.
1The little pads along this edge are only about 0.15 to 0.2 mm apart: right at PCB Power's published limit. The design allows it only because the fab says it can make it, and only on this chip.
Real data: core3588.kicad_dru
The rule in CORE3588's design-rules file that allows it.
How it works
Where it fits
DRC enforces the hard limits; DFM is the judgment on top. The fab's CAM team checks DFM again and raises an EQ if something looks unbuildable.