Invoices may consist of numerous invoice numbers (provided you group invoices by matter), matters, and billing items.
A payment can be applied to an Invoice Number (which means it will be for all billing items of the same Matter) or the whole invoice, but cannot be applied to an individual Billing Item. If an invoice is grouped by matters, it is possible to pay off specific invoice numbers and the billing items associated with the invoice number. As a result, different billing items, within the same invoice, may have different paid dates.
To make it easier to determine when an invoice and its individual parts were paid, each Invoice and each Billing Item has a paid date (as of April 2016). Invoices are considered paid when the balance is zero. Thus, partial payments to a whole Invoice don't provide paid dates to any of the billing items. Likewise, if an Invoice is not grouped by matters (thus does not have individual invoice numbers), then all the underlying billing items will have the same paid date as the Invoice itself. To see the paid date for a Billing Item, enter the Billing Item record (by clicking on the item in the Invoice Preview or in the Billing Items table).