there are two types of system that each band can be in:
Local
National
the national competitions include things like the area, pontins etc. and are supposed to keep bands from all over the country of the same standard in the same section. As there is a bias to the north of good bands, the championship sections are fuller in the north, and emptier in the south.
the local ones are run by the local committees (usually). Like the SCABA contest at brighton/hove and folkstone, and are intended to keep bands local and mean there is no requirement to travels hundreds of miles.
sections wise you have 5 sections in the national system, and normally 4 in the local system. They are each individual 'leagues'. However if you enter a 'local' competition from a different area then you can enter in the same section you are in your own area.
In the olden days the national system used to be champ, 2, 3, 4, before the 1st section was added, so often the 1st section is added to the championship section if there are only 4 sections (at both championship and local contests).
clear? :dunno
some of this may not be entirely correct or just plain wrong, but as far as i know, it is correct.