Sentence:  Even when <WSD>probouleutic</WSD> discussion was opened up to a larger number of individuals - as on 6 October - meetings continued to operate independently of the privy council. Restricting <WSD>probouleutic</WSD> debate to a small group of personally selected councillors was particularly appropriate for an issue - Elizabeth's marriage - which she defined as one of the "arcana imperii" ('mysteries of state') reserved for her own judgement qand on which she had sought to restrict debate in council, parliament and public debate since her accession. 
Meaning of the word marked with <WSD> tags:  (by extension) Pertaining to preliminary discussion before discussion in council, parliament, or other public debate.