Dimension is an option of several functions and denotes the space-time dimension. Possible settings are: 4, n, d, D, ... ,the variable does not matter, but it should have Head Symbol.
The dimension of the indices is not shown by default but can be inspected easily.
Setting the global variable $LorentzIndices to True will display the dimension (if different from 4) as a subscript.
|The FeynCalc Book||DeltaFunctionPrime||DiracGamma|