you're talking modal harmoney, which don't have a "proper" dominant, as you recall, for natural minor the V is minor, and so the "dominant" for natural minor is the VIIb .
it is called a "cadence" function.
for example:
if we are on C lydian(F# on scale), we got I, then the II will be major chord, and is a strong cadential function. you should avoid going from II7 or II to IIIm or V because that is a ionian movement.
the #IV-7b5 is an avoid chord since it sound also like a dominant of the ionian scale.
it is also common to use the root of the tonic mode (lydian in our case) as a pedal point, and move the other triads accordingly to emphasize the modal quality.
for example |Bb | Bb | Bb | Bb | C/Bb | C/Bb | C/Bb | C/Bb |
improvising over the sound of lydian:
IIIm going to IV and the lydian scale is over the IV.
IIm6 >IIIm