Staff Presets are essentially sound variations for the instrument. A variation might indicate a certain size of instrument, or instruct the musician to use a particular playing technique or tool - such as mallets or sticks. It could also provide additional information about the sound you're after - for example, whether a snare drum is to played with the snare wires on or off.
A Staff Preset looks like a boxed text instruction, and it informs both the real-life musician, and the sampler within StaffPad, about the specific sound you're after.
Only some instrument staffs within StaffPad contain Presets. The ones that do will always feature real recorded variations behind them, so you'll hear the effect of them during playback.
Staff Presets are rendered as boxed text above the staff. If you tap on that boxed text, a pop up will appear and show you the available variations, or presets. Tap a preset to switch the sound.
To change a staff preset, long touch on the bar you want to change, and choose "Change Preset". Tap the desired preset to add it into the score.
You can change a preset as many times as you like on a staff, and you can drag the box around to change the position of the preset change.
Staff Presets are a powerful way of customising the internal playback variation of a staff, whilst also leaving a usable and useful score marking for musicians too.