In general, the relation between a voltage and a current is described by an impedance.
As far as the blocks are concerned, each internal port is associated with an input impedance, Zb, that generally depends on the frequency:
Zb=Zb(f).
To obtain a minimal reflection at port b for a certain frequency f0, the Impedance, Zi,of the conductor connected with it must equal its input impedance:
Zi(f0)=Zb(f0).
See also