Tight or Loose Coupling in Directional Couplers

Hi my Friends

what means Tight or Loose Coupling in Directional Couplers?
is it try to clarify the Nominal Coupling Factor (ie. 3 to 20 dB as Tight and above 20dB as Loose)
or it try to clarify the Frequency Sensitivity (Ripple) in the Bandwidth?

Thanks in Advance

It is referring to the nominal coupling factor, but is not a precise term i.e. no hard line at 20dB.

Tight and loose couplings refer to the value of the coupling level between coupled ports or coupled lines.
For example if you want to design a tandem coupler you need two pairs of coupled lines with coupling level of -8.34 dB. Then -6 dB is tighter and -10 dB is looser than -8.34 dB.