Q:

In VBA : Dim a, b as String ==> only b is a String. Can I get a reference to this behavior?

In a VBA module in Access 2013 I've been bitten by this: 
Dim a, b as String

a is a Variant, b is a String
Where can I get a reference to this behavior?
On the MSDN page "Dim Statement (Visual Basic)" I see the following example:

   Dim a, b, c As Single, x, y As Double, i As Integer  
   ' a, b, and c are all Single; x and y are both Double

which is not the same behavior.

A:

You are looking in the wrong documentation. VB is not VBA.
From the VBA documentation:

You can declare several variables in one statement. To specify a data type, you must include the data type for each variable. In the following statement, the variables intX , intY and intZ are declared as type Integer.
Dim intX As Integer, intY As Integer, intZ As Integer 

In the following statement, intX and intY are declared as type Variant only and intY are declared as type Variant only intZ is declared as type Integer.
Dim intX, intY, intZ As Integer 

You don't have to supply the variable's data type in the declaration statement. If you omit the data type, the variable will be of type Variant.

