SFAD[{{q1 +..., p1 . q2 +...,} {m^2, s}, n}, ...] denotes a Cartesian propagator given by \frac{1}{[(q_1+\ldots)^2 + p_1 \cdot q_2 ... + m^2 + s i \eta]^n}, where q_1^2 and p_1 \cdot q_2 are Cartesian scalar products in D-1 dimensions.
For brevity one can also use shorter forms such as SFAD[{q1+ ..., m^2}, ...], SFAD[{q1+ ..., m^2 , n}, ...], SFAD[{q1+ ..., {m^2, -1}}, ...], SFAD[q1,...] etc.
If s is not explicitly specified, its value is determined by the option EtaSign, which has the default value +1.
If n is not explicitly specified, then the default value 1 is assumed. Translation into FeynCalcI internal form is performed by FeynCalcInternal, where a SFAD is encoded using the special head CartesianPropagatorDenominator.
SFAD[{{p, 0}, m^2}]\frac{1}{(p^2-m^2+i \eta )}
SFAD[{{p, 0}, {m^2, -1}}]\frac{1}{(p^2-m^2-i \eta )}
SFAD[{{p, 0}, {-m^2, -1}}]\frac{1}{(p^2+m^2-i \eta )}
SFAD[{{0, p . q}, m^2}]\frac{1}{(p\cdot q-m^2+i \eta )}
SFAD[{{0, n . q}}]\frac{1}{(n\cdot q+i \eta )}