The Picture Transfer Protocol (PTP) facilitates remote operations on a camera connected computer. Typical operations are uploading/downloading photos and remote captures. Most cameras support some basic operations over PTP, but often only in a limited fashion. For example, downloading of files can be limited to just JPEG files. If you want simple control over the camera without USB a protocol connection (for example, using a micro-controller or RC equipment) see USB Remote. Note the USB remote option must be disabled to use the PTP Extension. Related forum thread: CHDK PTP interface