Disable_dir_dep values other than 'Y' and 'N'

While researching another issue, I found that we have several values other than 'Y' and 'N' for disable_dir_dep field (Pay_Earnings).  I thought the only valid values were 'Y' and 'N'.  In addition to those values we also have 'M', 'A' and 'Q'. So, how does the payroll system handle the 'other' values (I am assuming that they will be treated like 'N')?  Also, what about blank (spaces) in the field....how does the system handle that value?  Please clarify.  Thanks!