Optional parameters can be left out by the caller and take a default value. IsMissing tests Variants that were omitted.

Code

Function Tax(amount As Double, Optional rate As Double = 0.07) As Double

• Default value

  Function Tax(amount As Double, Optional rate As Double = 0.07) As Double Tax = amount * rate End Function → Tax(100) uses 7%; Tax(100, 0.1) overrides

  Defaults in the signature.

• Optional object

  Sub Log(msg As String, Optional ws As Worksheet) If ws Is Nothing Then Set ws = ActiveSheet → An optional object tested with Is Nothing

  Objects default to Nothing.

• IsMissing

  Sub Run(Optional v As Variant) If IsMissing(v) Then ... → IsMissing works only for Variants

  The Variant form.

• Optional parameters must come after required ones.
• ParamArray takes any number of arguments as an array.
• Named arguments (rate:=0.1) skip earlier optionals.

• IsMissing is always False for typed optionals; they get their default.
• Optional with no default gives 0 or "" for typed parameters.