BrickUnderground

Over the years, almost every apartment in my co-op has been combined with another apartment.  Some have been renovated.  None has ever had the combination or renovation filed with DOB.  Now the co-op’s attorney says they all have to be filed, but only when being sold.  This doesn’t seem to make sense.  Shouldn’t every apartment, irrespective of whether or not it’s being sold, be filed?  Also, being relatively new to NYC, I don’t know what is meant by “filed.”  Can anyone direct me to the relevant laws/codes concerning this?