I'm sure there exists a better document at explaining provisioning, but Exchange 2007 ActiveSync policies should provide you with an idea of the wide range of security policies an Exchange admin can a demand a mobile device follow in order to sync. A provisionable device is one that can, and will, follow at least one or more of those policies when asked to do so.

Please note though that nothing requires a provisionable device to support all those policies listed in the document above. In fact, as I understand it, many provisionable devices only support a small subset of all the possible policies. Therefore, it is quite possible to have a provisionable device that's rejected by your employer's Exchange server.

A non-provisionable device, on the other hand, simply blows a raspberry at the Exchange server as soon as the server begins listing demands. This, of course, leads to an immediate severing of the ActiveSync connection.

vitaly_repin, a MfE developer with Nokia, has previously provided examples as to why provisioning is not supported on the N900. Mostly it seems to boil down to Nokia taking a position of "if you're going to do it, do it right", which is the hardest, longest path to follow.