When selecting, sort cards by: When the user is prompted to select specific cards from the deck, the cards can optionally be sorted (alphabetically) using the listed property. Leave blank to list cards by their curring position in the deck. Example: cards in a deck can use a Marker trait to specify a card number (001,002, etc.) and always list cards in order of their assigned number.
The "when selecting, sort cards by" option sorts the items in the deck in the order that they're in the deck by default. To change it, you need to sort it by a property that each item possesses.
For example, if you wanted to sort a standard playing card deck alpha/numerically, you would have to give a property to each card that ranked them in the correct order, let's call it Rank. Since "Ace of Clubs" is alphabetically before "Ace of Diamonds," you would give the Ace of Clubs the Rank property of "001" and the Ace of Diamonds Rank 002. You'd have to go through each card in the deck and assign it a unique rank. Once done, you can enter "Rank" into the "sort by" box, and it will sort the cards in that deck by that property.
CAVEAT: I've never made use of this feature, that's simply my understanding from reading the documentation.