The simplest durable lesson here is this: vacancy is not a one-off event; it is an ongoing cost that compounds when tenants cluster. Three quick checks before you act: 1. Name the mechanism in plain English: Every month a unit sits empty costs rental income, increases per-unit fixed costs and may pressure the landlord to accept worse lease terms. 2. Say why it matters for behavior or portfolio decisions: That matters because optimistic occupancy assumptions are behind most real estate investment disappointments. 3. Set the review question: If you had to teach this without jargon, what would you tell someone to monitor first? In real life: A commercial property with three tenants whose leases all expire within 12 months faces a correlated re-leasing risk that most pro formas understate. Common slip: The mistake is underwriting 95% occupancy on a building that has historically averaged 88% without explaining what changed. The point is not to memorize the label. The point is to know what variable is actually doing the work.