Number of Versions (Iterations) to keep

I remember there was a way to control the number of Version (better iterations) that PPT keeps.

When users work with automatic synchronization they get a big number of iterations very soon.

There was a way to limit this number of iterations but I cant remember.

When you save the next Iteration and the number is reached, the lowest iteration is deleted.

I also ask if I find the way to configure the number of Iteration to keep, will this effect existing files ?

I suppose not, so is there a way to "purge" those files ?