definitely not trying to argue here - maybe mash comes in different forms in different parts of the country. The mash I buy from the local mill has whole oats with the hulls on, corn kernals, some soybeans, etc and some of it is ground up.- you can pick up a handful and see the oats, corns, etc. There is also a nutrient mix added... the only pellets I've seen are uniform color (kind of tan) and if you crush them with a spoon, you get powder.
With pellets, the ingredients are pulverized to flour consistency, and formulated in a machine - "baked" to make the pellets.
My banties eat the mash same as my other chickens. I did buy a bag of pellets by mistake one day, and the chickens won't eat them indoors. Sometimes if I throw some on the ground, they will pick up some of them to eat. But they leave most of them.
I've been trying to use up the 50 pounds by throwing a handful in the feeder when I fill it, but they mostly pick them out and leave them.
Some folks feed only pellets, and some folks don't have access to anything else - pellets are more likely to be carried at the big stores. So if you live in an urban area, you are likely to use pellets.
Have to say, chickens will do fine on mash or pellets, or whatever.