Some confusion about the "Simple convection in a quarter of a 2d annulus" parameter file

 Hello everyone, I have some confusion in the document "Simple convection in a quarter of a 2d annulus". 
 The first question is why we only set the values of the thermal expansion coefficient and viscosity of the material in the material model here, unlike the reference density, reference specific heat capacity, reference temperature and heat conductivity of the material which are set in "Convection in a 2d box". 
 The second puzzle is that we only have the name of the gravity model but we don't have a value for the acceleration of gravity