This online calculator, just like Isogram checker, is related to isograms. According to wikipedia, isograms can be useful as keys in ciphers since isogram sequences of the same length make for a simple one-to-one mapping between the symbols. Ten-letter isograms like PATHFINDER, DUMBWAITER, and BLACKHORSE are commonly used by salespeople of products where the retail price is typically negotiated, such as used cars, jewelry, or antiques.
For example, PATHFINDER cipher, P represents 1, A represents 2, and so on. The price tag for an item selling for $1200 may also bear the cryptic letters FRR, written on the back or bottom of the tag. A salesperson familiar with the PATHFINDER cipher will know that the original cost of the item was $500 so that if the price is negotiated, he will not accidentally eliminate all of the 240% margin in the $1200 price shown to prospective buyers.
So, this calculator takes entered letters and tries to match them against 10-letters English isograms, taken from here, with PATHFINDER, DUMBWAITER, and BLACKHORSE being the first to check.