You can choose to view completed items on your prioritizer. They’ll appear with a checkbox and line through them.

You can also choose to have completed items drop from your list.

Adjust what you see and don't see on your prioritizer using the the sort icon at the bottom of your screen, in the advanced options menu, and then selecting an option from under show/hide.

If you like seeing items disappear from your prioritizer as you complete to-do's, toggling hide done + snoozed will take items off the prioritizer once you've marked them done.

Toggling everything means that an item you've marked done will simply be crossed out but remain on your prioritizer until it resets for the next day:

Finally, toggling only "musts" hides everything but the items you've tagged with #must from your prioritizer.