What is a no-fee journal?

Spontaneously, I would have answered: an OA journal which does not charge publication fees. Further consideration might be needed, however, as DOAJ and QOAM come to different conclusions with respect to a number of journals. For example, the Elsevier journals 1873-2445 and 1873-1562 or the Springer journals 1029-8479 and 1434-6052 do not charge authors because libraries have paid the bill as members of the SCOAP3 project. But does that make these journals ‘no-fee’ as DOAJ seems to conclude, or are they just ‘discounted journals’ as QOAM claims? The outcome may have relevance to the wider Plan S debate as well.

I would appreciate if the community could shed some light on this.

Leo Waaijers