This option is the simplest and is often referred to as the "Maximum Utility Rule." It assumes the respondent will choose that product with the highest overall utility. All other less preferred products receive zero share for this individual. The results for the First Choice model are invariant over many kinds of rescalings of the utilities.
If two or more products are exactly the same in terms of utility (and more preferred than any other product in the simulation scenario), the respondent's choice is split evenly among them.