Selecting records while you have groupings, will only select the records of the groupings that have been expanded and that is currently displayed on the page. Varibill does not allow the selection of records that are not being displayed. This allows you to select multiple records across groups, without selecting all records of collapsed groups. Varibill furthermore only allows the emailing of closed invoices to ensure invoices not yet finalised are not sent to your customers.
For more information on the basic functionality of Varibill, invoice statuses and emailing invoices, refer to docs.varibill.com.