From publisher blurb:
The Pick 'n' Mix consists of small PDFs, each for the price of $0.49. Each contains, for example, a single monster, an NPC, an item, and so on. The idea is that you you purchase the assorted items you desire and put them together into a binder to make your own "house rules" book.
Pick 'n' Mix items are all formatted identically, so that your binder will look neat and tidy. Different types of item will have a different colour border (monsters are brown, spells are blue, magic items are gold, and so on). That way, your binder is colour coded neatly.
Pick 'n' Mix is a license, and publishers are able to use the license to create their own Pick 'n' Mix products for sale at the EN World GameStore. The license includes trade dress and formatting standards, etc. It also includes the use of the Pick 'n' Mix logo.