I have always misinterpreted this idea.
I thought it referred to the rumination of ideas... Don't put notes you just made in it (from any source, like reading a book), instead let it ruminate.
I was wrong. I realized this when chatting with Gemini Advanced.
But either way, the DeepL translation of this paragraph:
"Slip box as a clarification pit - don't just put clarified notes in it. Postponing reviewing and deciding - also a question of speed."
It is moreso related to the idea of fleeting notes and unprocessed ideas. Have to think a bit more about what Luhmann meant. Maybe @chrisaldrich knows something.