It’s a method of selecting a portion of a mailing list in set numerical pattern, such as every fifth name or every tenth name, to produce an exact random quantity. The nth select is designed to produce a representative cross-section of a list.
Most list management software programs have a function which will automatically select the names at random based on desired x output quantity. If you can trade some statistical validity for simplicity, a quick way to do nth select from any list is to sort by 3rd letter of first or last name. Then export a copy of the first x number of records. Remember to make notes of the method used so that on future orders you can omit (avoid) the names previously provided. Or just use copy of your prior list to de-dupe from any future list creation.