This object is the filename of a file downloaded onto the controller card's non-volatile storage, which can include an explicit device or path prefix. This is the actual filename the controller card matches against when deciding which stored file to push down to a service module, so setting it incorrectly, or to a file that does not exist on the controller's storage, is a common cause of a subsequent download failing with fileOpenFailed. Setting csDefineFileName to slot0:ids-sig-update.pkg tells the controller exactly which stored signature-update package to send when the download operation is triggered.