Since the area of the planing surface constantly varies, it's common to just use BWL * LWL as the reference area.
Units: Usually kilograms per square metre, or pounds per square foot. One might think of bottom loading in terms of pressure, but that can lead to confusion- so I prefer to avoid using units of pressure (Pa, or N/m2) to describe bottom loading. Just think of it as another arbitrary ratio that can be used to compare boats.
What it's used for
Bottom loading is a very useful figure when trying to determine how well a hull will plane. Higher bottom loading indicates that more lift must be produced by a given size of planing surface, so the boat will have to run faster or at a steeper trim angle to get on plane. We must confirm, of course, that the hull shape is amenable to planing- straight buttocks, roughly prismatic afterbody, etc.- before this ratio will be of much use.
In general, lower is better, within reason. A boat whose bottom loading is much less than her competitors' will be quicker to pop onto plane and better at low to moderate speeds, but may have a bouncy ride in rough weather.
Less than 100 kg/m2 (20 lb/ft2) - This implies a very light hull that should pop up on plane quickly at low speeds.
About 200 kg/m2 (40 lb/ft2)- For boats in the 4 to 7 metre range, this is the point where getting up on plane needs a bit of effort, and there may be a range of "no-go" speeds where the boat plows around with its bow in the air. Once up, planing can be easily sustained.
About 300 kg/m2 (60 lb/ft2)- For a boat in the 4 to 7 metre range, overloading is indicated, and the boat probably won't plane very well. Given enough power, it may run OK at high speed, but will be a dog from 8 to 20 knots or so. In boats from 10 to 15 m LOA, bottom loadings in this range are typical and indicate reasonable performance.
About 400 kg/m2 (80 lb/ft2)- Bottom loadings in this range on a 5 to 10 m boat indicate almost no ability to plane. On a 10 to 15 m boat, this bottom loading would indicate slight overloading and poor low-to-moderate speed performance, and on a 20 m boat would imply typical, but unremarkable, planing performance.