The Unique Parameter box gives you several options for naming your files. You can choose to add a unique character by using Numbers, Letters, or Dates. You can also choose to Increment, Decrement, Don't Change, or Random the values.

Numbers - numbers would be something like 0001 or 0002

Letters - This could be something like a, b, c, or you could make it more complex like aaaa, aaab, aaac, aaad, etc...

Dates - This allows you to place a date in your filename