How is the default value for the Charge Billable flag determined?

Some charges are by default billable, and some are by default not billable. How is this determined?

A default charge's billing status (Billable/Not Billable) is determined by the contract type linked to the charge:

1. All new charges that are linked to System or Global contracts are set to Billable.
2. All new labor charges that are linked to "Block of Time" contracts are set as Not Billable, while expenses and parts charges are Billable.
3. All new charges that are linked to "Block of Tickets" or "Block of Money" contracts are set to Not Billable.