To use a deed, open the satchel, select the deed and activate it; then whistle, and the chosen horse will come running. Marston's previous horse, if any, will wander off.
Despite appearing under "Consumables," a horse deed can be used an unlimited number of times.
The player can hold as many deeds as they can buy.
Using the Deed is the fastest and easiest way to save the game, and unlike the other methods it doesn't cost 6 game hours.
A deed can be bought at any General Store in the single player. (Price depends on fame and honor)
Since all two-star horses are effectively equal in game terms, and Marston can whistle for a random two-star horse at any time, there's no particular reason to buy a two-star deed except for cosmetic appearance, other than the fact that having one (or preferably two) deeds enables quicksaves.
One-star deeds are complete wastes of money unless playing Hardcore mode (where horses do not respawn after your steed is killed or lost). After riding in into the next town is recomended to either buy another horse or steal one for no cost.