What do the authors mean by "closing operator"?

Question

I'm reading a paper titled "Luminescence spectra of quantum dots in microcavities II Fermions" (link). In section III, the authors introduce so-called closing operators. I don't have enough background knowledge to understand the technique there. So it's natural for me to look for other literature or books on the topic. But it turns out the "closing operator" is defined only by these authors and I couldn't find any literature on this.

Could you give me any key words which can lead me to related literature? Or you can point me directly to a literature.

Answer 1

After searching I find these "closing operators" defined by the authors correspond to particular sets of operators mentioned in a part (on quantum regression theorem) of the book Statistical methods in quantum optics 1.