I came across a great article by Neil Grimes on the BMC website. Please see extract below.

The system for grading traditionally protected climbs in BMC (and MI) guides is the traditional, two-part British grade, a combination of the adjectival and technical grades.

Adjectival grades

The adjectival grade is the first part of the grade, and attempts to give a sense of the overall difficulty of a climb. This will be influenced by many aspects, including seriousness, sustaindness, technical difficulty, exposure, strenuousness, rock quality, and any other less tangible aspects which lend difficulty to a pitch. It is an open ended system, and currently runs from Easy to E11. Along the way, and in ascending order, are Moderate (M), Difficult (D), Hard Diff (HD), Very Difficult (VD), Hard Very Difficult (HVD), Severe (S), Hard Severe (HS), Very Severe (VS), Hard Very Severe (HVS) and Extremely Severe, the last category being split into E1, E2, E3 etc.

As with all grades, these catagorisations are subjective; there are no cut off points. VS runs smoothly into HVS, HVS runs into E1. Also, some climbers are better at safe, technical routes, some better at bold easy ones. Some climb well on delicate slabs, some on overhanging fist cracks.

Technical grades

The second part of the grade, the technical grade, is there to give an indication of the hardest move to be found on the route. They come onto the scale somewhere around 4a and currently run thus; 4a, 4b, 4c, 5a, 5b, 5c, 6a, 6b, 6c, 7a, 7b. It is an open ended scale, although while climbs continue to get harder and harder, this is usually reflected in the E grade, with climbs tending to become more serious and more strenuous rather than more technical.

Combined grades

Going back to the combined grade, you should see how the combination of these two grades goes to suggest the difficulty of a climb, and what type of difficulty this might be. As a help, climbs of a particular adjectival grade, will often have an associated average technical grade. Roughly these are S, 4a; HS, 4b; VS, 4c; HVS, 5a; E1, 5b; E2, 5c.

If in relation to a particular adjectival grade, the technical grade is high, (e.g. VS 5a, E1 5c) then you can expect the route to be technical in character, with maybe a single, hard, well protected move. If the technical grade is low for the adjectival grade, (e.g. HVS 4c, E3 5b) then expect either a very sustained and strenuous struggle, or a route with relatively easy climbing, only in a serious situation. Which one of these two it might be can hopefully be determined by looking at the climb. (i.e., an overhanging hand crack could reasonably be supposed to be sustained and well protected, a blank slab could be supposed to be serious.)

It is best used in conjunction with the downloadable grade comparison table. This also shows conversions between various bouldering grades.

Please click here to see the full article on the BMC website.