A smaller cap inside your overall sum insured that applies to one specific type of claim (for example, cruciate ligament injuries).
A smaller cap inside your overall sum insured that applies to one specific type of claim, for example, commonly cruciate ligament injuries, which are expensive and common enough that many insurers cap them separately.
A sub-limit means that even if your overall annual limit is high, one condition might still only be covered up to a much lower amount. The fewer and smaller the sub-limits, the less likely a single serious condition is to blow through its own cap before your main limit is reached.
Some insurers go further and apply a universal “per-condition cap”. This is single dollar figure that applies to every condition claimed, not just one or two named ones. That's a materially bigger real-world risk than a single named sub-limit, since it caps every large claim you might make, regardless of what it's for.