Scratch means something that initiates "scratching" of the ground by the chickens. Traditionally, whatever the farmer or mill keeper had left in odds and ends for grains, got swept together and sold for "chicken feed" thus, the expression, it's cheap, it can be had for "chicken feed", or it is expensive, as in "it's not just chicken feed".
Since people love to watch their chickens scratch around and peck for feed and it occupies them, the stores began to make and sell "scratch". There is no set formula. But the nutrition tag on the bag will tell you a lot. Run-of-the-mill scratch has only half the nutrition and normally, no mineral package that feed has. Given that scratch is now $10 a bag most places, it isn't a great bang for the nutrition buck, imho.
It can be used to entice the chickens to gather, to follow you, etc. It has that purpose, but nutritionally, it has very little purpose. It's entirely up to the flock keeper to make these choices about feed for his/her flock. At these prices, I no longer buy it.