The option controls the counting rule. Y counts SO and SI within mixed strings as one each. N does not count the SO and SI and is the default.
DBRLEFT('ab<.A.B>',4) -> '<.B>'
DBRLEFT('ab<.A.B>',3) -> '<.A.B>'
DBRLEFT('ab<.A.B>',4,'Y') -> '<.A.B>'
DBRLEFT('ab<.A.B>',3,'Y') -> '<.A.B>'
DBRLEFT('ab<.A.B>',8) -> ''
DBRLEFT('ab<.A.B>',9,'Y') -> ''