In the UK there are various tests for flammability of fabrics and these tests vary in their applicability to the domestic/home market and to contract markets eg hotels, hospitals and so on.
When fabric is treated to a crib 5 standard it is suitable for most uses. It is particularly used in contract environments like hotels where the impact of a fire can be serious. Crib 7 is a more stringent test, so fabric that passes this test is suitable for more hazardous environments like prisons or offshore installations.
During the fabric testing a frame is made with the fabric spread over it. The bigger the frame the more it is exposed to the flame. And of course the frame is a called a crib. And the Crib 7 crib is bigger than the crib 5 crib! So the Crib 7 crib is exposed to a more severe flame.