What distinguishes a submerged-versus-submerged kill?

Did a Dutch submarine also sink an enemy submarine?

The claimed distinction concerns both boats being underwater; attacks on surfaced submarines are a separate category.

The Dutch example is considered a submarine firing from underwater at a Japanese submarine on the surface, with the exact incident left uncertain. Such attacks are distinguished from a kill in which both attacker and target are submerged. The latter is the category being claimed for the British V-class action.