Pick a brick
Pick a brick (PaB or PAB) is a term used by LEGO to denote
- an area of a brand retail store, or theme park, or the online store where bulk elements can be purchased
- the bulk brick itself, after is purchased.
PaB is usually sold by volume in brand retail stores, but in theme parks it is sometimes sold by weight. The online store sells by piece.
In physical retail locations the elements are usually arranged along a wall in clear plastic bins. Each bin usually has only one element type/color in it, although occasionally, (when one runs out) there may be mixes of two or more.
There is a "PaB cup" used in many locations, it comes in two sizes and you fill it up as full as you can and still get the cap on, and then pay a fixed price for that cup. At other locations you fill a ziplock bag instead and it has to seal. The PaB cup and bag use a pattern of many circles in a rectangular grid in their markings, which is evocative both of the studs on elements, and of the wall itself since the circular bins are arranged in a rectangular grid as well.
Many ALEs have done analysis of the most efficient way to pack certain elements to maximise how many fit into a cup, and in some cases posted results. Certain elements (such as 1x1 tiles or 1x2 tiles, when they appear) are highly prized as filler as they fit within the spaces left by bigger elements.