Hi,
The SALES & INVOICING section in task form has three options Billable, Products on Tasks, and Extra Quotations.

If the timesheets and material need to be invoiced to the customer, enable the Billable option. To track products consumed during the task, enable the Products on Tasks option. Then, if the user needs to add an extra quotation, then enable the  Extra Quotations. 

Please go through our blog 'How to Manage Field Service Tasks in Odoo 16'

https://www.cybrosys.com/blog/how-to-manage-field-service-tasks-in-odoo-16https://www.cybrosys.com/blog/how-to-manage-a-field-service-task-for-your-project-in-odoo-16

Hope it helps