VBA supports 2 different concatenation operators, + and & and both perform the exact same function when used with String types - the right-hand String is appended to the end of the left-hand String.
If the & operator is used with a variable type other than a String, it is implicitly cast to a String before being concatenated.
Note that the + concatenation operator is an overload of the + addition operator. The behavior of + is determined by the variable types of the operands and precedence of operator types. If both operands are typed as a String or Variant with a sub-type of String, they are concatenated:
If either side is a numeric type and the other side is a String that can be coerced into a number, the type precedence of mathematical operators causes the operator to be treated as the addition operator and the numeric values are added:
This behavior can lead to subtle, hard to debug errors - especially if Variant types are being used, so only the & operator should typically be used for concatenation.