FeynAmp[q, amp] is the head of a Feynman amplitude,
where amp denotes the analytical expression for the amplitude and q is
the integration variable. FeynAmp[q1, q2, amp] denotes a
two-loop amplitude. FeynAmp has no functional properties
and serves just as a head. There are however special typesetting rules
attached.
This is a 1-loop gluon self-energy amplitude (ignoring factors of 2 \pi).
FeynAmp[q, GV[p, \[Mu], a, q - p, \[Alpha], c, -q, \[Beta], e] GP[p - q, \[Alpha], c, \[Rho], d] GV[-p, \[Nu], b, p - q, \[Rho], d, q, \[Sigma], f] *
GP[q, \[Beta], e, \[Sigma], f]]\int d^Dq\left(f^{bdf} f^{ace} \Pi _{ef}^{\beta \sigma }(q) V^{\mu \alpha \beta }(p\text{, }q-p\text{, }-q) V^{\nu \rho \sigma }(-p\text{, }p-q\text{, }q) \Pi _{cd}^{\alpha \rho }(p-q)\right)
This is a generic 2-loop amplitude.
FeynAmp[Subscript[q, 1], Subscript[q, 2], anyexpression]\text{FeynAmp}\left(q_1,q_2,\text{anyexpression}\right)