rule!xyz: if(origcdt.firstname = changecdt.firstname, {},"firstname compare

1. What is the rule input type. It should be of type any type.
2. Instead of passing cdt directly to the rule, pass CDt1.first name,CDT2.firstname

you should use "ri!" (for rule input variables) or "local!" (for rule local variables) as prefix to every variable as per variable type.
eg: apply(rule!xyz, merge(ri!origcdtlist,ri!changecdtlist)) if origcdtlist and changecdtlist are rule inputs.

It's worth knowing that plain text comparisons executing in the expression editor may give a different result to those executing in process as they may or may not be case sensitive.

I would use fn!exact() to compare the strings. What is the result you are getting?

Hi Conorc, Need to compare multiple fields in CDT. I got the error like "Invalid Array"as it is not possible to merge cdt using fn!merge. I tried below one and it fixed my issue apply(rule!xyz, 1+enumerate(length(ri!changecdt),ri!changecdt,ri!origcdt).

It would have been simpler to have a rule with inputs of origFirstName and changedLastName, and then do something like apply(fn!xyz,merge(origCdt.firstName,changeCdt.firstName)) - assuming the arrays are the same length.

I suggest you read the documentation carefully for fn!merge (it doesn't simply append the arrays together) - as well as for fn!apply, giving particular attention to the difference between the array and context inputs.

In rule use underscore like below.
Apply(rule!xyz(_,_),merge(input1,input2))