ByVal or ByRef - User Defined Types

When you declare a variable with a User Defined Type it can only be passed in using ByRef.
A user defined type is a value data type.
When you declare a variable with a User Defined data type it can only be passed in using ByRef.

ByRef

Passing a User Defined Type into a subroutine using ByRef allows the original variable to be changed.

Type udtType 
   Field1 As String 
End Type 

Public Sub Passing_UserDefinedTypes1() 
   Dim myUserDefinedType As udtType 
   myUserDefinedType.Field1 = "before" 
   Debug.Print myUserDefinedType.Field1   'before  
   
   Call ByReference(myUserDefinedType) 
   Debug.Print myUserDefinedType.Field1   'after  
End Sub 

Public Sub ByReference(ByRef theUserDefinedType As udtType) 
   'this changes the original
   theUserDefinedType.Field1 = theUserDefinedType.Field1 & "more" 
   
   'this changes the original
   theUserDefinedType.Field1 = "after" 
End Sub 

ByVal

Passing a User Defined Type into a subroutine using ByVal is not possible
If you try and declare a user defined type parameter that is passed ByVal you will get a compile error.

Public Sub ByValue(ByVal theUserDefinedType As udtType) 
End Sub