It is a protected method. It is not supposed to be called directly.At most, you can call it in a subclass if you decide to extend NotORM_Result.
Now, for your answer, it expects a column name (or columns separated by comas). Accepts both the dot and colon notation.
What it does is generate an array with the " LEFT JOIN `table` ON `parenttable`.`somecolumn` = `othertable`.`othercolumn`" string that NotORM will use later.