What's the difference between fog, fog banks, and fog patches?
Thanks to mins's link, the Australian Government Bureau of Meteorology explains everything in detail. To summarise the particular part I was asking about:


FG on its own means fog (visibility &lt; 1000 m) covers more than half of the horizon
PRFG and BCFG are fog covering less than half of the horizon
PRFG (fog bank) is more extensive than BCFG (fog patches)


Another way of looking at it is like cloud cover descriptors. FG is like BKN or OVC: more than half the horizon is covered. PRFG is SCT, and then BCFG is FEW.

All the above are used when the temperature is above freezing point and the fog reaches more than 2 metres above ground. If the fog is only below 2 metres agl, it's shallow fog MIFG instead. If the temperature is below freezing, so the fog consists of supercooled water droplets, it's freezing fog FZFG.