IPB has a concept of permission sets, which are set uniquely on every forum that is created.
XenForo on the other hand starts with a base of user group permissions, and then adds custom forum permissions on a per-group basis if required. If no forum permissions are defined for a particular user group for a particular forum/node, the user group permissions are used.
This leads to a dilemma as to how to import these permissions.
On the one hand, we could import all the permissions verbatim, but this would mean that every user group would have specific permissions set for every forum. While this is possible, it it not the way that XenForo is intended to be used, and may make ongoing management difficult.
Alternatively, we could do a more simple interpretation of the IPB forum permissions and limit the translation to private / public switches and then allow the admin to go through the forums and set up specific permissions as they required.