Results of a mask
All (entire) columns in your selection will have this mask applied to them. You may also notice that entries in columns with masks will be displayed in blue text.
NB: Applying a mask to a column that is not numeric will not result in any change in its display apart from the blue text. It will, however, for cloned rows, affect the modified field's validation process as ACE will take into account the active mask when validating the new value.