13th Age Ritual question – expenditure?

In the book, it states these are the steps of casting ritual magic:

  1. Choose the spell that will be used and expended by the ritual.
  2. Tell the GM what you are trying to accomplish and gather necessary ingredients for the ritual.
  3. Spend 1d4 minutes/quarter-hours/hours (as determined by the GM) preparing and casting the ritual. You can’t cast other spells during this period. A PC taking damage won’t necessarily end the ritual, but it will be ruined if a character falls unconscious or launches an attack of their own.
  4. Make a skill check using one of your magical backgrounds and the ability score the GM deems appropriate. Use the standard DC targets (or a special DC set by the GM), depending on your tier and the results you’re hoping for. The higher the level of the spell consumed by the ritual, the greater the effect. No matter the outcome, the spell is expended until your next full heal-up.

My question is, What does "expended" indicate? A cleric gets a certain number of spell slots per day and selects what spells are in those slots. I am assuming ritual magic is being used on spells not selected for those slots, or rituals wouldn’t be required. So what exactly is being "expended?" Does it just mean that you can’t do another ritual of the same spell until full heal-up? Can you perform a ritual for a spell that you have already expended with a normal skill slot? (an extra cure wounds when you have already cast it from a slot?) I love how open ended this is, but the language in the book is really hard to parse at times.