The breakout board provides a way to access all lines on the conan inter-board connector via standard 2.54 mm strips. They can be accessed using standard socket boards. Own boards can be added to the breakout board using standard 2.54 grid standard pcb boards. Pin numbers on the breakout board refer the pins of the connector. This way e.g. own sensor boards can be added to the particle. As these own sensor boards are sandwitched over the particle (the Particle resides between your board and the breakout board) total space for such devices will still be small.
The breakout board can be reduced in size from 50x42mm to 38x33mm by cutting away the part with the battery holder and the power-on switch. To cut the board back do the following: