What separates a bulkhead from armour?

What is the difference between a bulkhead and armour?

Bulkheads are designed chiefly to contain water pressure, while armour is designed to resist weapon impacts and is usually far thicker.

A watertight bulkhead must withstand hydrostatic pressure and preserve the boundary of a compartment. Armour has to resist a specified level of hostile fire and is therefore normally much thicker and heavier. The two can both contribute to structural strength, but they are sized for different loads and failure modes. This distinction prevents the damage-control discussion from treating every strong internal plate as armour.