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Use conditional calls and callbacks for implementation-specific functions. For example, to provide additional data from the cash register during a transaction, like loyalty data, you use the Additional Data Callback. If the cash register does not handle these calls and callbacks, they will block further processing of the transaction.

Optional calls and callbacks do not need confirmation from the POS system. For example, the progress callback does not require any action. 

These optional extras allow you to integrate specific features from the library: