The Unit Weight in the Define Material Properties dialog works in conjunction with the Initial Element Loading, and is only applicable if Body Force is included in the Initial Element Loading. That is, Unit Weight is only enabled if Initial Element Loading is either:
Field Stress & Body Force or
Body Force Only
Unit Weight is disabled if Initial Element Loading = either Field Stress or None
If applicable, the Unit Weight is used to determine the Body Force applied to each finite element.
Unit Weight vs. Unit Weight of Overburden (Gravity Field Stress)
If you are using Gravity field stress, you should understand the distinction between the Unit Weight of a material entered in the Define Material Properties dialog, and the Unit Weight of Overburden entered in the Field Stress dialog.
The Unit Weight of a material in the Define Material Properties dialog, is used to determine the Body Force applied to each finite element of a given material type.
The Unit Weight of Overburden in the Field Stress dialog, is used to determine the Gravity Field Stress for all finite elements in the model.
For a multiple material model, these two unit weights will not necessarily be the same.
For a single material model, then the Unit Weight in the Define Material Properties dialog should be the same as the Unit Weight of Overburden in the Field Stress dialog.