DTS and Dolby Digital are audio formats that can consist of 5 discreete channels of audio plus an LFE channel (5,1). THis isn't to say that all audio encoded as DTS or Dolby Digital will consists of this many channels of audio though. Pro Logic is an upmixing mode that creates pseudo channels in order to create pseudo surround from non surround sources or sources that are missing the full complement of discrete surround channels. If you've engaged the Pro Logic II Surround DEcoder mode then the receiver will apply this processing to any incoming audio that doesn't comprise of 5.1 or more channels of discrete audio. If for example the source was a 2 channel AAC file then the receiver would apply Pro LOgic to it to create pseudo 5.1, but if the source was encoded with Dolby Digital or DTS 5.1 then the Pro LOgic processing would be bypassed because it would not be required because the sources are already 5.1 in nature.
You can disengage the Pro Logic processing mode by pressing the STRAUGHT mode button on the remote Pressing it again will reinstate the mode. You can change the Pro LOgic mode to another mode by pressing the SUR DECODER button on the remote. Use this button to cycle through the varios modes available to you.