The flow velocity is normally set to a known value. At walls, the velocity is usually set to 0. Also, turbulent quantities such as turbulent kinetic energy and dissipation rate can be set to fixed values which have to be estimated first.
The pressure is often set to a known value on outlets. In incompressible simulations, it is common to use a value of 0 for the Gauge pressure.
Temperature or heat transfer rate are often set to known values