Basic Oracle Concepts and Programming Question:

An IN condition is single value again a list of values. It returns TRUE, if the specified value is in the list. Otherwise, it returns FALSE.
Some examples are given in the script below:

SELECT CASE WHEN 3 IN (1,2,3,5) THEN
'TRUE' ELSE 'FALSE' END FROM DUAL;
TRUE

SELECT CASE WHEN 3 NOT IN (1,2,3,5) THEN
'TRUE' ELSE 'FALSE' END FROM DUAL;
FALSE

SELECT CASE WHEN 'Y' IN ('F','Y','I') THEN
'TRUE' ELSE 'FALSE' END FROM DUAL;
TRUE