In my application i have several actions that the user can perform like : displaying items, search items, purchase items.. there isn't any unique sequence which the user should follow specifically to finally perform the checkout. So i wonder how i can represent all these actions through messages all along a unique life cycle of my system and i don't know if i can just and simply choose any order for them but i don't think it will be a good and suitable representation of my user-system interactions if i cannot at least indicate that the order of these actions can be performed randomly by the user.
Multi-threading representation in a sequence diagram cannot be solution for my problem because these actions aren't supposed to be parallels.