If I remember rightly, it may be relevant in the following circumstances:
1. Some domestic treatment plants can be configured\installed so as to produce a relatively clean effluent which meets the standard for being released direct to a watercourse.
2. Sometimes, due to site conditions, it might be necessary for a domestic treatment plant of this type to be used ( i.e. unavailability of a percolation area).
In such circumstances, a discharge licence ( permit?) should be obtained.
I had to research this before, but it was a long time ago, so rules may have changed.