Since the Space Arcanum’s purview includes portals, doors, and similar liminal spaces, it’s only natural that many Space spells allow the mage to specify a Key — a particular item carried, password spoken, or other criterion that either activates a spell or bypasses its effects.
For example, a portal might only open on the summer solstice, or a hidden path might be visible to anyone of the mage’s bloodline.
Spells that can be Keyed are called out in their descriptions, usually at the cost of a Reach.
The Key may have a number of requirements equal to the spell’s Potency.