Arguments are passed ByRef by default, so a procedure can change the caller's variable. ByVal passes a copy, which is safer when the procedure should not affect its inputs.

Code

Sub Bump(ByRef n As Long)
  n = n + 1
End Sub

• ByRef

  Sub Bump(ByRef n As Long) n = n + 1 End Sub ' after Bump x, x is one larger → The caller's variable changes

  ByRef shares the variable.

• ByVal

  Sub Show(ByVal n As Long) n = n + 1 End Sub ' the caller's x is unchanged → A copy is changed, not the original

  ByVal protects the caller.

• Force ByVal

  Call Bump((x)) → Extra parentheses force ByVal for one call

  A quirk worth knowing.

• Use ByVal for values you only read; use ByRef to return several results.
• Objects are always references; ByVal on an object only protects the variable, not the object.
• Event signatures dictate ByVal Target As Range; do not change them.

• Accidental ByRef changes callers' variables mysteriously.
• Parentheses around a single argument in a Call silently switch to ByVal.