Invoice Processor is a specific sub-role of Controller that can be granted to users in Liquid.
Separately, an Invoice Processor is also anyone that should be able to add additional approvals on any given invoice, as well as remove those additional approvers. Any approvers added on a specific invoice can be removed, but approvers added automatically via Approvals Settings cannot be removed from specific invoices. Companies usually add their AP person, accountant, or bookkeeper as an Invoice Processor.
When an invoice is received from your Vendor, the Invoice Processor can elect to get copied on emails as Invoices go through the approval/rejection process — or they can choose to save their inbox and opt-out of these emails. The important thing to note is that the Invoice Processors aren’t required to do anything to move an invoice along, though some Invoice Processors will want make sure that the correct invoice information is in the system (vendor, date, amount, chart of accounts).