How should the name "Kats" be treated when it precedes a(nother) surname?
For example, should "Yosef Kats Shenfeld" be established as
 
          (1) Shenfeld, Yosef Kats (as if a 2nd forename)
          (2) Shenfeld, Yosef,$cKats (as if equivalent to "ha-Kohen")
          (3) Kats Shenfeld, Yosef (as if the 1st element of a compound surname)
 
(1) can never be correct.  
The choice between (2) and (3) depends on the availability of other evidence.
Where it's known that the man's son calls himself Shelomoh Yitshak ha-Kohen Shenfeld, there's evidence for treating "Kats" as equivalent to "ha-Kohen," as in (2) above.
If no evidence like this is available, we feel the logical treatment is as in (3)-- as a compound surname.