I think it's called the Law of Propagating Nulls.
There used to be a pretty good wiki on it at UA but cant find anything there anymore.
Basically as Gicu points out Anything + a null is null.
It's the difference between using & or +
if you had rs!FirstName & " " & rs!MiddleName & " " & rs!LastName and there was no middle name you would have extra spaces using the "&"
if you have (rs!FirstName + " ") & (rs!MiddleName + " ") & rs!LastName and there was no middle name (it is a null) there would only be one space between first and last name as a (null + " ") is null so it would not include the second space.
You enclosed each part in parenthesis so the expression is evaluated together as a pair.
In your case as mike points out it would be CustomerName & (vbnewline + AddressLine1) & (vbnewline + AddressLine2) & (vbnewline + AddressLine3) & ... etc.
If this helped, please click the star * at the bottom left and add to my reputation- Thanks