Four triggers
A workflow starts when an invoice is created, when its state changes, when a watched field changes, or time-based after the invoice or due date.
Workflows
Workflows describe how invoices and receipts travel through the company. Each can be limited to specific invoice types and switched active or inactive.

What is included
Define what should happen to an invoice automatically, triggered by creation, a state change, a field change or time.
A workflow starts when an invoice is created, when its state changes, when a watched field changes, or time-based after the invoice or due date.
Every execution is recorded as a run with status and a per-node log, so it can be read back exactly what happened to an invoice.
Invoices carry an assignee field for the person responsible. It can be set manually, shown as a column, and filled automatically by workflows.
More in the same area

Invoisary. Invoices, expenses, and budgets in one workflow.