TextBox has a specific meaning which doesn't appear to be what you are looking for in this instance.
Try this:
Set up your document with two Plain Text Content Controls. Set the title of one to "Input Number", set the other title to "Status"
Add this macro to your ThisDocument module
Code:
Private Sub Document_ContentControlOnExit(ByVal ContentControl As ContentControl, Cancel As Boolean)
Dim aCC As ContentControl, sMsg As String
If ContentControl.Title = "Input Number" Then
If IsNumeric(ContentControl.Range.Text) Then
If CLng(Trim(ContentControl.Range.Text)) > 100 Then
sMsg = "In Spec"
Else
sMsg = "Out of Spec"
End If
Else
sMsg = "Needs a number greater than 100"
End If
For Each aCC In ActiveDocument.SelectContentControlsByTitle("Status")
aCC.Range.Text = sMsg
Next aCC
End If
End Sub