A very fine grid of candidate points (25 to 100 microns) the router searches across, layer by layer. Option A for "seeing".
How it looks
Drawing: an idealised patch of board
The router lays a fine grid of candidate points over the board. Drawn here at a few points per part so you can see it; on CORE3588 a 50 micron lattice would be about 1,700 x 2,000 points per layer.
How it works
Where it fits
The lattice is option A of how a router "sees" the board, the first of the three dials. It is half of the hybrid method; the other half is the exact geometry check.