Sets the properties of the selected seat.
In Populate, a seat is a box on which a simulated person "hangs out," either sitting by themself or interacting with other seated people. Visible features include connecting lines that show a group of people talking, and markers that show the character's spatial orientation and gender: red for female and and blue for male.
Turning on Single can change the behavior of nearby characters, who might also behave as Single if there is no one else to interact with.