Hart protocol implementation
On the web site it also says


  HART Protocol Specifications
  
  The HART Protocol Specifications (HCF_KIT-13) are sold as a collection
  of over 15 documents that specify the different aspects of the
  protocol and test procedures for slave devices. The specifications are
  sold as a kit that include all the documents in the specification set.
  The cost of these documents is $975.


The royalty free bit just means that you can sell the devices without having to pay a royalty or licence fee for the privilege.  You just have to buy the specifications.