It is a line for faster checkout because the number of items in your basket has to be 12 or less. The cashier has less work to do for each person.
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GPYIn the UK, signs such as these have been a point of argument, with fussy customers complaining that they should read "12 items or fewer". I think one supermarket even changed their signs because of complaints.See e.g. http://languagelog.ldc.upenn.edu/nll/?p=552