PotButton creates a macro button, called Pot, that allows you to set four different items to it. Using the button will have your character use the item either set with no modification or the item attached to when you're pressing ctrl, alt or both.

To easily change the item you have tied to the key, pick up the item on your pointer, then hold shift while pressing the alt or ctrl key (or both). If you'd like to change the item you have tied to when you're pressing ctrl or alt, hold shift and ctrl or shift and alt at the same time as pressing the key while you have the item picked up on your pointer.

Additionally, you may use /potbutton to bring up the config menu (also available via Interface -> AddOns -> PotButton), which allows you to not only set the item to be used by the button combinations, you may also set backup items, should the item not be available in your inventory at the time.

Resetting your character's items back to default can now be done with the slash command /potbuttonreset. Do not worry, this will only affect the currently logged in character's items and not any items on other characters. The default has now been set up with the entire list of potions, in order of strength, to be used as the non-modified click, so immediately the strongest potion your character has in his or her bags will be the item the button will use.

Notes

Due to restrictions Blizzard has placed, it is not possible to change a macro key while in combat. Changing the item attached to your PotButton macro key while in combat will be reflected upon leaving combat.
If your key bound to your PotButton's key is on the number pad, holding shift while pressing this button will instead press the non-number-lock version of that key rather than the number. You will need to then use the key that is on the number row of your keyboard.
Programmatically changing the macro buttons while the normal Blizzard macro UI window is open is mildly buggy, so when updating the Pot button while that window is open, the addon will momentarily close the window and then reopen it. Visually, this will probably go unnoticed, but you will hear the sound of that window being closed and reopened.