This is how shift schedules are determined at my job

Exact words from communication about this below. 5 years or less: Will be scheduled shifts where the system determines the need is. 5 and 6 years: You'll be asked to provide a 2 hour timespan for your shift start time preference. The start time of your schedule will fall within the 2 hour timespan you indicate. 7 to 9 years: You'll be asked to provide a 1 hour timespan for your shift start time preference. The start time of your schedule will fall within the 1 hour timespan you indicate. 10 or more years: Will get their preferred shift.