Is there any fees to pay to use the VDDK in commercial software ?

Hi,

While searching for more information I found  the Technology Alliance Partner (TAP) Program  page on VMWARE website.

This is unclear to me. Please explain whether there is any compulsory fees to pay in order to use the VDDK API  for developing COMMERCIAL or FREE Backup and Replication Software for VMWARE vSphere ESXi.

Thanks.