A batch is a set of records derived from a larger collection of records. In Salesforce, when using the bulk API, data jobs are divided into batches. Each batch is sent to the server in an HTTP POST request. Each batch is processed independently by the server, and not necessarily in the order it is received. Batches may be processed in parallel mode (concurrency) or serial mode (no concurrency). It is left to the client's discretion to decide the batch size, but the default is 10,000. Using a smaller batch size may cause an organization to use more of their allotted bulk api batches per day than is necessary.