Determines the tab order of the weblet on the form. The tab_index property value determines the tab order as follows:
1. Objects with a positive tab_index are selected in increasing tab_index order (and in source order to resolve duplicates).
2. Objects with a tab_index of zero or blank (the default) are selected in source order.
3. Objects with a negative tab_index are omitted from the tabbing order.
Blank. The weblet is selected in source order.
Blank or a valid numeric value.