How are you defining regrettable losses in turnover?

We define a regrettable loss as a loss for the company; indeed those people who have the highest performance and potential but also your trusted employees who might not have the highest potential but who have a specific expertise that will be lost or that are very reliable and deliver in times of crisis and who just get things done. What is key to found out is what the manager could've done to prevent the person from leaving; is it a once in a lifetime opportunity the employee is accepting, or is the culture, job content, missing challenges or anything. You can never prevent people from leaving but try to detect early signals for those who will be a regrettable loss and make sure you feel you've done what you could even when then leave. 

Hi - We'd typically use the outcome of our talent process to determine this. We use a (relatively) standard 9box to assess potential/sustained performance and the top boxes would determine whether it's regrettable or not.