You create the following function that serves as the decision making function for the symbols:

create or replace function DDMADMIN.DECISION_FUNCTION(
     I_INST_ID varchar2,
     I_USER_ID varchar2
) return number
is
begin
     if (length(I_INST_ID)+length(I_USER_ID)) > 10
     then
      return 1;
     else
      return 0;
     end if; 
end;

The function name is DDMADMIN.DECISION_FUNCTION. The function returns 0 or 1 based on whether the stored procedure returns sensitive data.

In the DefMaskRSSym security rule, you add MATCH_FUNCTION and MATCH_FUNCTION_PARAMS symbols. The MATCH_FUNCTION symbol value is the name of the function, DDMADMIN.DECISION_FUNCTION. The MATCH_FUNCTION_PARAMS symbol value is the number of arguments in the unmasked stored procedure that you want to pass to the decision making function. In this example, the symbol value is 2 and Dynamic Data Masking passes the first two arguments of the stored procedure to the decision making function.

The following image shows the rule action of the DefMaskRSSym rule: