Coupling coefficient of microstrip resonators
In a coupled resonator filter, all coupling has to be correct, though we often treat it in two stages. The first stage is to get the inter-resonator coupling right, as this is often fixed by the fabrication. The second stage is to get the input and output coupling right, as this is often tunable, and can be assessed by measuring the S11 and S22 of the filter while tuning it. 

The reason the two sets of coupling are often treated differently is that the inter-resonator coupling tends to come out of the theoretical design process about right, just being the geometry of similar sections. However the change in geometry from transmission line to the first resonator, particular the ground geometry, tends to make it difficult to get it right by design, hence the need for tuning on the prototype.

I once designed a comb-line filter milled from alli, where the 'tuning' consisted of sending it back to the machine shop to have another 0.2mm skimmed off the base and tooth tips, so that the tapping hole on the end teeth 'moved up' the tooth. You might call this 'destructive tuning'.