Questions re: Driving Key?

I cretaed a Link and BimlFlex define the HK_LNK as the Driving Key. My questions are:

What pattern are used to determine the Driving Key? What does HK_LINK as a Driving Key mean??

The driving key is used to control the updating/end dating of Link Satellites for driving key relationships.
        
This is documented more here: https://docs.varigence.com/bimlflex/concepts/driving-keys

You should be able to set the column that is the driving key in the Link using the change type (e.g. one of your HK_<name> columns).

The only way that BimlFlex stores the Driving Key information is in the attached LInk Satellite, and only when the Link Satellite is using end dating. In that scenario, the LSAT record that is not active anymore is end-dated by the load SP.