12mm doesnt have many applications indoors unless its a warehouse. You might use it directly over the till or prep area, this would give you a very small field of view. conversely 2.8mm is very wide - almost fisheye it will give you a good coverage of the whole room from the corner but with limited detail. 4mm is regularly used for shop coverage, 6mm for an aisle.
Bear in mind the narrower your field of view the more you lose in the foreground.
Another example:
Bear in mind the narrower your field of view the more you lose in the foreground.
Another example: