FCLoopCanonicalize[exp, {q1, q2, ...}, loopHead] is an auxiliary internal function that canonicalizes indices of multi-loop integrals with loop momenta q1, q2, ... that are wrapped with loopHead. The output is given as a list of 4 entries, of which the last one contains a list of all the unique loop integrals in the given expression. After those are simplified, the original output of FCLoopCanonicalize together with the list of the simplified unique integrals should be inserted into FCLoopSolutionList to obtain the final replacement list that will be applied to the original expression.
FCLoopCanonicalize[myHead[FVD[q, \[Mu]]], q, myHead]\left( \begin{array}{c} \;\text{myHead}\left(q^{\mu }\right) \\ \{\text{FCGV}(\text{cli191})\to \mu \} \\ \;\text{myHead}\left(q^{\text{FCGV}(\text{cli191})}\right) \\ \;\text{myHead}\left(q^{\text{FCGV}(\text{cli191})}\right) \\ \end{array} \right)
FCLoopCanonicalize[myHead[FVD[q, \[Mu]] FVD[q, \[Nu]] FAD[q, {q + p, m}]] + myHead[FVD[q, \[Rho]] FVD[q, \[Sigma]] FAD[q, {q + p, m}]], q, myHead]\left\{\left\{\text{myHead}\left(\frac{q^{\mu } q^{\nu }}{q^2.\left((p+q)^2-m^2\right)}\right),\text{myHead}\left(\frac{q^{\rho } q^{\sigma }}{q^2.\left((p+q)^2-m^2\right)}\right)\right\},\{\{\text{FCGV}(\text{cli201})\to \mu ,\text{FCGV}(\text{cli202})\to \nu \},\{\text{FCGV}(\text{cli201})\to \rho ,\text{FCGV}(\text{cli202})\to \sigma \}\},\left\{\text{myHead}\left(\frac{q^{\text{FCGV}(\text{cli201})} q^{\text{FCGV}(\text{cli202})}}{q^2.\left((p+q)^2-m^2\right)}\right),\text{myHead}\left(\frac{q^{\text{FCGV}(\text{cli201})} q^{\text{FCGV}(\text{cli202})}}{q^2.\left((p+q)^2-m^2\right)}\right)\right\},\left\{\text{myHead}\left(\frac{q^{\text{FCGV}(\text{cli201})} q^{\text{FCGV}(\text{cli202})}}{q^2.\left((p+q)^2-m^2\right)}\right)\right\}\right\}
FCLoopCanonicalize[myHead[FVD[q1, \[Mu]] FVD[q2, \[Nu]]], {q1, q2}, myHead]\left( \begin{array}{c} \;\text{myHead}\left(\text{q1}^{\mu } \;\text{q2}^{\nu }\right) \\ \{\text{FCGV}(\text{cli211})\to \mu ,\text{FCGV}(\text{cli212})\to \nu \} \\ \;\text{myHead}\left(\text{q1}^{\text{FCGV}(\text{cli211})} \;\text{q2}^{\text{FCGV}(\text{cli212})}\right) \\ \;\text{myHead}\left(\text{q1}^{\text{FCGV}(\text{cli211})} \;\text{q2}^{\text{FCGV}(\text{cli212})}\right) \\ \end{array} \right)