The verification in BiSSAP, no matter how it is invoked, uses the same set of rules.
Some rules are format-specific. ST.25-specific rules are applied when a project is imported.
When the program is about to perform an export operation, it verifies the project using rules determined by the export type. That is why it may be possible to export a file, even if normal verification reports errors. By the same token, it may not always be possible to export a project in a given format, even if the verification reports no errors.
XML is intended to be the main format in BiSSAP, so the standard verification is XML verification. This may result in the generation of error messages in situations which would otherwise be acceptable in ST.25.
The above remarks apply when BiSSAP verifies projects in its internal database. The batch verification, which works on external files (not imported), always uses rules suited to the file type in question.