Difference of Invoice Billable Customer and Bill Sales Orders

Invoice Billable Customer
Go to Transactions tab > Customers > Invoice Billable Customer

-When the Bill Costs to Customers feature is used, you can invoice the customer for billable expenses, items and time. Billable meaning the expenses incurred, items purchased and time spent on a particular job will be charged to the customer.

Bill Sales Orders
Go to Transactions tab > Sales > Bill Sales Orders

-Billing sales orders may create an invoice or a cash sale. This is billing the orders placed by the customer.