AttachPropertyInstanceEx should be used in cases where you cannot represent the data exactly as it appears in the frame. For example, the Ethernet frame length is not actually a field in the Ethernet frame, but it is a useful property to have. The parser writer for Ethernet can build a local word value representing the frame length and add it to the system with an AttachPropertyInstanceEx. Another example of when AttachPropertyInstanceEx becomes important is when the data does not fall into a byte or word. For example, the LLC Next Send and Next Receive correlators are bytes that do not use the first bit of the byte. To correctly represent this field, the LLC parser moves the data out of the frame into a local variable and shifts it to get the correct value. Then it adds the property with AttachPropertyInstanceEx. Another example of using AttachPropertyInstanceEx is when the data can be more easily used by the system in another format. See the Time example.