The "Multi-group" tab allows you to set multiple groups to the selected stock item. The main Reporting group 1 (Stock group 1) is selected on the Reports Reporting group 1 section of the Edit stock screen. The same list of the available Reporting group 1 (Setup → System parameters → Groups (Stock group 1 menu) will be listed and may also be linked to the stock item.

TurboCASH4 - Stock item Multi-group tab