StandardPropagatorDenominator[propSq + ..., propEik +..., m^2, {n, s}] encodes a generic Lorentzian propagator denominator \frac{1}{[(q_1+ \ldots)^2 + q_1 \cdot p_1 + \ldots + m^2 + s i \eta]^n}.
propSq should be of the form Momentum[q1, D], while propEik should look like Pair[Momentum[q1, D], Momentum[p1, D].
This allows to accommodate for standard propagators of the type 1/(p^2-m^2) but also for propagators encountered in manifestly Lorentz covariant effective field theories such as HQET or SCET.
StandardPropagatorDenominator is an internal object. To enter such propagators in FeynCalc you should use SFAD.
Overview, PropagatorDenominator, CartesianPropagatorDenominator, GenericPropagatorDenominator, FeynAmpDenominator.
FeynAmpDenominator[StandardPropagatorDenominator[Momentum[p, D], 0, -m^2, {1, 1}]]\frac{1}{(p^2-m^2+i \eta )}
FeynAmpDenominator[StandardPropagatorDenominator[0, Pair[Momentum[p, D], Momentum[q, D]], -m^2, {1, 1}]]\frac{1}{(p\cdot q-m^2+i \eta )}