IPX Coding
The letter X is used in any place in the code where specifying a digit is meant to be avoided. There may be various reasons for choosing this coding variant, such as marketing considerations. Thus, e.g. an IPX7 rating for a consumer device specifies that the device has water protection up to limited immersion, but gives deliberately no information as to whether the device has any protection against mechanical ingress or dust. Among other common IP ratings using the letter X are IPX4. IP2X is frequently used on electrical items to specify the item must prevent finger access to live terminals i.e plug sockets are IP2X.
Read more about this topic: Ingress Protection