I think that's put there by RSLogix 5000 to indicate a placeholder when there's a math error. You sometimes also see #NaN for "Not a Number" or #INF for an Infinite result.
It's definitely not a user-entered tag or shorthand.
To Ken's point without seeing the statement #QUAN could refer to Quotient error, are you dividing by zero? or Quantity error, maybe it is expecting a non negative Integer and it is not getting one