These ridged proving baskets, produce a ‘baton’ or ‘bloomer’ shaped loaf with markings that are similar to those imparted by the cane type of basket. They produce that distinctive ‘Continental’ appearance without the problems of sticking from which cane baskets sometimes suffer.
The sizes indicate the weight of a typical bread dough that each basket will accommodate, allowing for expansion during proof.
Which shape to choose? It's a matter of personal preference, of course. Factors which may be worth considering:
it is generally possible to fit more long-shaped loaves than round ones in a given oven space
some people find baton shaped loaves easier to cut and the resulting slices more 'regular'.