A single-name limit says: no more than X% of equity in one ticker, or no more than N risk units. Common settings are 5-10% of capital and 2 units of risk for active traders.
Its purpose is to survive the case you cannot foresee: an accounting fraud, a failed trial, a halt that reopens 40% lower. Stops and analysis do not protect against a trading-halt followed by a gap, and the only defence is size.
Enforce it at the idea level too. If a name gets to the limit by rising, that is fine - trim if you want, but the risk is now largely in open profit. If it gets there by adding, you have overridden the rule. Write the limit into the sizing spreadsheet so it is checked automatically rather than remembered.
Related: concentration-risk, position-limit, max-open-risk, unit-sizing