Here's the exemption being applied to several pages of the record:
21. The head of a government institution may refuse to disclose any record requested under this Act that containsWhile the provision isn't as explicit as it could be, the exemption generally refers to consultations or deliberations involving government policy (Canadian Council of Christian Charities v. Canada (Minister of Finance),  4 FC 245 at para. 30, 32):
(b) an account of consultations or deliberations in which directors, officers or employees of a government institution, a minister of the Crown or the staff of a minister participate,
Despite the importance of governmental openness as a safeguard against the abuse of power, and as a necessary condition for democratic accountability, it is equally clear that governments must be allowed a measure of confidentiality in the policy-making process. To permit or to require the disclosure of advice given by officials, either to other officials or to ministers, and the disclosure of confidential deliberations within the public service on policy options, would erode government's ability to formulate and to justify its policies.In contrast, personal matters are covered by the separate exemption under section 19 (also applied to much of the Senate document request).
On the other hand, of course, democratic principles require that the public, and this often means the representatives of sectional interests, are enabled to participate as widely as possible in influencing policy development. Without a degree of openness on the part of government about its thinking on public policy issues, and without access to relevant information in the possession of government, the effectiveness of public participation will inevitably be curbed.
In other words, by the Privy Council's own account, communications between some combination of Harper, the PCO and Duffy about the Senate expense scandal were dealt with as a matter of government policy rather than mere personal (or indeed partisan) discussions. Which further discredits the "Nigel Wright acted alone" and "I couldn't have known" lines of spin from Harper - while raising all the more need for an accurate account of Harper's own participation.