Crib 7 is a larger crib than Crib 5. Sorry that didn’t help did it? It is the right answer but I’ll explain!
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.