For better understanding,

•  Paste the following data from the clipboard:

 Col1;Col2; A;1; A;; C;1; C;; 

• Type the following Add calculated column expressions:
  
  My_Col:

   Case  when ([Col1]='A') and ([Col2]=1) then "X" when ([Col1]='C') and ([Col2]=1) then "Z" else "Dont print" end

 Case   when ([Col1]='C') and ([Col2]=1) then "Z"  when ([Col1]='A') and ([Col2]=1) then "X" else "Dont print" end

• The below output is displayed with Null result in one case, "Dont print" in the other case, where the order of the when clauses should not matter.

• In Spotfire, null is treated as false, in all but the last condition. This is considered a defect.
• For now, a workaround is to add an additional dummy statement  as the very last WHEN condition, to make the ELSE clause work as expected. In the example below, "when false then "dummy"" was added for this purpose.

 Case when ([Col1]='A') and ([Col2]=1) then "X"  when ([Col1]='C') and ([Col2]=1) then "Z"  when false then "dummy" else "dont print" end