Primary Value = The actual main/real value of the dimension ... as opposed to the tolerance or dual dimension values.If you used the Insert > Model Items method to place the dimension in a drawing view, changing the Primary value will change the model size.
Override Value = A manually entered value which replaces the real Primary value to a "pretend" value. Creates a "not to scale" dimension. It will not change the model size. (IMO, this option should not exist)
Reverse Direction = Exactly that. It changes the direction by 180°.
Create a simple rectangular plate
Create a drawing of the plate
Use the Insert > Model Items method to insert the dimensions
Select a dimension
Experiment with the options