This article explains the difference between paying invoices automatically from the general/trust retainer versus paying them manually.
To change this setting go to your matter > billing (calculator) and scroll down to Retainer. You will see a dropdown menu next to pay new invoices from retainer. You can then select Automatic or Manual.
Some jurisdictions do not allow you to process a payment from trust against a bill until a number of days have passed. If you fall under such restrictions, then you will not be able to apply a trust payment to an invoice either manually or automatically.
Automatic means that payments to invoices will be made automatically once the invoice is saved and approved if there are funds in the general/trust retainer.
Manually means that payments will not be automatically made to invoices where there are funds in the retainer - payments will need to be processed manually.