BLE packet structure

Question

Please consider the images bellow, which can also be found here and here. The images represent the structure of a BLE packet, and the units used are bytes. The question I have is regrading the Par/Pay section of the packet. The maximum amount of user data that can be contained in a packet is 20 bytes (as cited in multiple sources), so what are the remaining 2 bytes in that section used for?

image description

endnode · Accepted Answer

Well in general you seems to be showing message on top of LE ATT layer where each packet has OP code (what Attribute "method" name is used in this packet) and 2-byte ATT handle (over which record in ATT Server table you communicate). This gives you minimal payload on top of ATT/GATT as 23 - 1 - 2 = 20B. You can see additional explanation at slide 239 and further of LE introduction presentation freely available (upon free registration) at BT SIG pages (see this post to get there, your second picture in the question above is "stolen" from there;). 
 Cheers Jan