The 'SELECT 1 FROM cln00' (1) query is almost equivalent to 'SELECT * FROM cln00' (2). The main difference is that when you use (2), the '*' will be expanded and attached to the list of columns, as for the (1) it will simply avoid to bind useless data to the main SELECT query. This is a normal procedure because the (NOT) EXISTS condition tests do not need any columns to return the boolean result. Thus (1) is faster than (2).
Maybe you will understand the query like that, which is equivalent to yours:
WHERE ctlnme IN (
SELECT ctlnme FROM clt00
SELECT ctlnme FROM cln00 )
Actually you are selecting all the cltnme from clt00 that aren't in cln00.