Environment
Situation
We periodically seem to be getting this "policy linkage" warning dialog on our 3.5.1 IDM servers. Usually when assigning a new server to a driver set. In all cases the server "IS" running IDM 3.5 or later - 3.5.1 Engine has already been installed on the servers. When it occurs removing the server a adding it back to the driver set gets rid of the warning do not a big deal.
Resolution
Starting with IDM 3.5 a new way of storing how policies are linked-in to the different policy sets was added. In the past IDM used to have the "DirXML-NextTransformationRule" attribute on every policy and stylesheet object which was of DN syntax. This was a huge limitation because each policy could only be used once in the middle of a "chain" or multiple times at the end of a chain. Moving one policy from one policy set into another brought over the whole remaining chain. With IDM 3.5 the policy linkage has changed. There is now an attribute called DirXML-Policies on a driver. It is of syntax path and contains a list of policy linkage information. There are no chains anymore and each policy can be used in many places and can be moved without possible side effects.
What that error message is telling you is that you have drivers that use new policy linkage in a driver set that has at least one pre 3.5 server in it. This server will not be able to run this driver as is.
Designer should convert back and forth between old and new linkage based on the server version you set in your Designer project. However, there are configurations that cannot be represented with legacy policy chains.