Bulk APIs are used for quickly loading large quantities of master data with a single API call. While the standard REST APIs only deal with a single object at a time, the bulk APIs are designed to accept a collection of objects to be processed at the same time. The bulk REST APIs only support the PUT method.
Even though the bulk APIs can handle a collection of objects in a single call. Each record is updated in a separate transaction. This means, that during a failure some of the records may have been updated while others may have not.