How can I specify the condition "is missing" for a case value in dialogs and SVB?
In selection condition (and in many other procedures) you can use IsMd(x) function.
In SVB you should read from variable definition MD code and then treat it as special value. Simple example
Dim s1 As Spreadsheet
Set s1=ActiveDataSet 'get actve data spreadsheet
If (s1.Value(1,1)=s1.VariableMissingData (1) ) Then MsgBox "Variable 1 case 1 is missing!"
In SVB you can also use:
If s1.MissingData(1,1) Then ...