Some installers did not like the Belden 7851NH cable that ETC specified for Cat 6 infrastructure prior to May 2009. The pairs separator could be difficult to work with. The alternate Belden 1872A was oval shaped and could be difficult to pull in conduit.
With these complaints, on the suggestion of an installer we switched to Belden 7881A, or 7882A for the Plenum version, for Cat6 installations. It is round and generally less expensive than the others. To further make it easier and cheaper for installers, ETC later changed the standards to "Cat 5e or better" and currently specify Belden 1583A.