Specctra Session — the routed result from Freerouting. KiCad imports it back into the .kicad_pcb to produce the final traces.
How it looks
Illustration: SES text written for this card from real CORE3588 track points
What a session file holds: just the wires. The highlighted path is the real copper of net Net-(Q1-B) on CORE3588, corner by corner (125.05, 39.74) to (128.75, 41.2) mm, 0.2 mm wide, written in SES units of 0.1 micron with the y axis flipped. Hand-written for this card; a real SES from Freerouting lists every net the same way.
Real geometry: CORE3588 copper, drawn from our board file (2D plan view)
The same wire as it sits on the board (amber).
1Start of the path: Q1 pin 1.
2End of the path: R59 pin 2.
How it works
Where it fits
The SES is the answer to the DSN's question. KiCad imports it to put the tracks onto the board, then DRC checks them.
Ants writes its result straight into the KiCad board instead, so it does not need this round trip.