The node+attribute property of a colocation constraints allows you to express the requirement, "these resources must be on similar nodes".

As an example, imagine that you have two Storage Area Networks (SANs) that are not controlled by the cluster, and each node is connected to one or the other. You may have two resources r1 and r2 such that r2 needs to use the same SAN as r1, but doesn’t necessarily have to be on the same exact node. In such a case, you could define a node attribute named san, with the value san1 or san2 on each node as appropriate. Then, you could colocate r2 with r1 using node-attribute set to san.