What is CP and AP and what are these options used for? There is this service menu where I can select different USB modes for my phone. One option is called CP and the other is AP.



What do the acronyms CP and AP stand for? I recognize MTP, PTP and ADB. But what are the options CP and AP used for?
TL;DR
Application processor (AP) =  CPU + GPU
Cellular processor (CP) = Cellular modem



The long version:
CP stands for Cellular Processor. It's also referred to as Baseband Modem. Another reference is Baseband Processor, or BP for short. AP stands for Application Processor. These are 2 of the 3 fundamental building blocks in mobile devices. The third being the Graphical Processor, or GP for short. The CP includes all the digital components required to communicate with a cellular network. The CP usually consists of an ARM-based processor and a DSP. It has its own operating system and communicates via a HS (High-Speed) serial connection with AP unit on which operating systems such as Android, iOS and Windows Phone are running.

Previously, CP, AP and (GP) were found in separate, dedicated chips. However, there is a trend to combine all three components on a single System on a Chip (SoC) to save cost, reduce power consumption, and shrink the overall size of the circuit board.

The table below is showing SoC chips with two AP — CPU &amp; GPU and one CP — the cellular or baseband processor. The table is copied from 3G, 4G and Beyond by Martin Sauter.





Bonus:
The USB settings in the image above is used to determine what drivers are enumerated when connecting your phone to the PC via USB cable. It is a multiplexer switch which determines whether the USB port is directly connected to the CP or the AP.

There are also options to select device features that will be enabled once connected to the PC. These options include:


MTP (Media Transfer Protocol)
ADB (Android Debugging Bridge)
PTP (Picture Transfer Protocol)
RNDIS (Remote Network Driver Interface Specification) for USB Ethernet
DM (Diagnostic Mode)
MIDI (Used to connect MIDI device such as MIDI keyboard)
Modem  



Click on image to enlarge  

More info can be found on XDA forums and in 3G, 4G and Beyond by Martin Sauter.



Reality:
AP mode of UART serial connection is favored upon.
Many blogs seem to have found that AP mode saves battery capacity,
charges phone faster when connected via USB, increases battery life.