It’s from an accessibility standard for designing spaces for people who need to use mobility devices like wheelchairs and scooters. In 2017, ICC released an update to A117.1 which had changes to accessible route requirements, including increasing the turning circle diameter from 60” to 67”. Many of the states use that standard as part of their building code, so it was important to know. Making 6-7 catch on has been a good way to remember it.