Sizing option you choose determines the refinement mechanism that the mesher activates to apply while meshing.

The Global Sizing Type in Files > Options...> Meshing > Meshing determines the available Sizing options for Mechanical Physics Preference.

When the Global Sizing Type is Program Controlled or On, the available options under Sizing group in Mesh Details view are:

When the Global Sizing Type is Off in the Files > Options...> Meshing > Meshing, Use Adaptive Sizing is Yes by default for Mechanical Physics Preference. For Electromagnetics Physics Preference,Use Adaptive Sizing is Yes. When the Physics Preference is set to CFD or Nonlinear Mechanical (in which case the default is Capture Curvature set to Yes ), or Hydrodynamics (in which case the sizing is uniform),Use Adaptive Sizing is No . The option you choose determines which refinement mechanisms are activated.

The available options under Sizing group in Mesh Details view are: