This topic will demonstrate how to export records from Acumatica ERP via the REST Contract-Based API. In contrast to the Screen-Based API of Acumatica ERP, the Contract-Based API provides both SOAP and REST interfaces. For more information on the Contract-Based API, see Acumatica ERP Documentation
actually its so simple: Opt-In approach says properties that are considered to be part of DataContract must be explicitly marked otherwise will be ignore, while Opt-Out means all of the properties will be assumed to be part of the DataContract unless marked explicitly.
This topic will demonstrate how to download files attached to a detail entity inside Acumatica ERP by using the Contract-Based API.