Those options allow you to toggle through different internal DRUM programs and assign midi output channels for external midi gear. Simply, you have a limit of 4 Drum program slots in the mpc to use within the same sequence at one time. You can only use one specific program per track but you can have different programs combining together within a sequence on separate tracks. You can also insert program changes mid sequence to add more sounds where one program is not in use. An example of a common program change would 2 ticks before the end of a sequence to change whatever program are assigned to either of the 4 program slots so that the next sequence to play has all it's programs assigned for a seamless transition (you can have 28 programs in total depending on the amount of ram available).
To change DRUM programs within a drum program slot..
load all your programs
hit shift 6
choose one of the 4 programs
scroll to the top where the program name is display and toggle left or right (you can also hit DRUM in the same screen for another display method of the same info).
To change which program is being triggered in the sequence window simply go beside the "S:" (think of the s meaning source) and change DRUM1 to another program slot (you can also change this to MIDI if you wanted to trigger just outboard gear using the device 1-16a & 1-16b as the midi channel output).
To insert program changes hit up the step edit window. If you are getting down with that kind of complexity (using more than 4 programs) then you will need to hit "open window" on the name of all your programs from the previous drum program menu to assign a pgm number for each of your programs in use (keep track of the numbers carefully). Now that you have program number assigned you can use step edit to insert program changes. If step edit isn't your thing there is an alternative. In the main sequence screen you will see a "PGM:" that is also to change what program is assigned to the current slot used in the track selected (the pgm number corresponds with the number you assigned earlier).
Here is another topic to check
http://www.mpc-tutor.com/phpBB2/viewtop ... drum#11578
Good luck!!