What does it give me for $5 per month?

Just guessing “Unlimited Talk, Text plus Data for 5 lines”

Select Choice plans were given to certain customer to replace very old, obsolete, plans that were holdovers from the various companies that merged into T-Mobile (i.e. PowerTel, Omnipoint, Aerial etc) These plans were available only to CSRs on the team that was making the conversion of the old plans. Regular CSRs couldn’t even see the details of the Select Choice plans, let alone have the ability to add one. I seriously doubt that you’ll be able to find a CSR today who’s able or willing to put you on this plan.