Plated holes cut in half at a module edge so it solders flat onto a carrier (Core1106). Needs enough paste to wick up.
How it looks
Drawing: CORE3588 has no castellated pads; its computer module plugs into board-to-board sockets instead
The edge of a module seen from above: each notch is half of a plated hole. Solder flows up the copper notch onto the carrier pad.
How it works
Where it fits
Castellated edges let a module act like one big part; the castellated pad card is the same idea from the connectors list. The Core1106 module uses them; CORE3588's module uses sockets instead.