SetDimensions is an option for ScalarProduct, CartesianScalarProduct and various FCLoopBasis* functions.
For scalar products it specifies the dimensions for which the scalar products will be set when ScalarProduct or CartesianScalarProduct are used with the equality sign, e.g. in ScalarProduct[a, b] = m^2. By default, the scalar products are set for 4 and D dimensions. By changing this option the user can add other dimensions or remove the existing ones.
In case of the FCLoopBasis* functions this option specifies the dimensions of the loop and external momenta to be taken into account when extracting the propagator basis.
Overview, ScalarProduct, CartesianScalarProduct.