What is the difference between Billable charges and Not Billable charges?

What is the difference between Billable charges and Not Billable charges?

A Charge can be set as Billable or Not Billable. When a Charge is set as Billable it means that you intend to bill the customer for this charge. The default value for this is determined by the type of contract under which the charge is created.

Note: Only billable charges can be billed.