What makes a warship a capital ship

When does a warship become a capital ship?

When it is a navy's principal instrument for fleet battle or major influence ashore, although battlecruisers can blur that functional definition.

A capital ship is a principal asset capable of deciding fleet combat or exerting major influence ashore. Battleships qualify through battle-fleet power and aircraft carriers through their ability to project force. A battlecruiser designed chiefly for reconnaissance, commerce protection, and hunting cruisers occupies a more ambiguous position. That ambiguity could be exploited in treaty language by arguing that such a ship was not intended to contest command of the sea against battleships. Operational purpose matters more than the visual impression of a very large warship.