California: Southern, Central, Northern?

I'm trying to download some 1:24000 topo maps for use in my mapping
program, Memory-Map. But their digital map shop interface is a masterpiece
of poor design. One (relatively minor) problem is that it splits
California into three separate maps: Southern, Central and Northern. But
doesn't provide any outline maps or definitions.

Are these formally recognised areas? If so, could someone point me to a
map clearly showing the boundaries please?

--
Terry, East Grinstead, UK