As you are probably aware, there are many different definitions of employee turnover. Unfortunately, each one comes with certain inherent weaknesses.
So we have the attrition rate (total turnover rate) : the total number of exits during the period, divided by the average number of people employed. Trying to make sense of such a raw number can be almost impossible. There's no clue as to whether this number is dominated by retirements, by dismissals, by redundancies, or by the loss of productive employees.
The retention rate: the percentage of employees that were employed at the beginning of the period and who remain with the company at the end of the period. Those that joined and then subsequently left, during the period of time you are looking at, get ignored by this definition.
Voluntary turnover: this time retirees and employees dismissed or made redundant, are not included.
Dysfunctional turnover: here the focus is on the loss of good performers. But the problem comes when you start to think about how this number is generated. How many departmental managers are going to admit to experiencing more than the occasional unwelcome departure, for fear of inviting an investigation into the reasons behind these exits?
Regretted turnover: similar to dysfunctional turnover.
There's no easy answer. No sensible way of discovering what you really want to know; which is how many people did we lose, that we would have liked to retain, and that given a few small changes, we would have been able to keep.
Some while ago I did write a short guide on employee turnover definitions. In truth it is probably best to track several different definitions, and to be cautious about benchmarking against other companies or even sector or industry figures.