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.

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