createDocument abstract method
Creates a custom entity document with specified parameters.
Builds a Document from explicit components (type, data, signature) for extended entity information.
@param docType - Document type (e.g., "visa", "bulletin")
@param data - Optional raw data content of the document
@param signature - Optional cryptographic signature for document verification
@return Custom Document instance
Implementation
Document createDocument(String docType, {
String? data,
TransportableData? signature
});