CartesianIndex is the head of Cartesian indices. The internal representation of a 3-dimensional i is CartesianIndex[i].
For other than three dimensions: CartesianIndex[i, Dimension].
CartesianIndex[i, 3] simplifies to CartesianIndex[i]. The first argument cannot be an integer.
Overview, LorentzIndex, ExplicitLorentzIndex.
This denotes a 3-dimensional Cartesian index.
CartesianIndex[i]i
An optional second argument can be given for a dimension different from 3.
CartesianIndex[i, D - 1]i
CartesianIndex[i, D - 4]i