In one sentence
The layered structure of debt and equity financing a property, from senior debt at the bottom to common equity at the top.
The longer version
The capital stack orders claims on a property's cash flow and value: senior debt first, then mezzanine, preferred equity, and common equity, each with its own risk and return. Where a piece sits determines its priority and price.
Lenders care intensely about what sits above and below them in the stack, because it defines their real position if things go wrong.
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