how DOCUMENT_CLOB gets populated in IBY_TRXN_DOCUMENTS table?

We have requirements to create custom payment documents. We are reusing ISO20022CGI.xsl file from Disbursement Payment File Formats. The data model has SELECT DOCUMENT_CLOB FROM IBY_TRXN_DOCUMENTS WHERE TRXN_DOCUMENT_ID =:TRXN_DOCUMENT_ID query. When I query DOCUMENT_CLOB, it shows empty. I want to understand if we have to fetch some information which is not available in the ISO20022CGI.xsl file how can we do it?

I have seen IBY_TRXN_DOCUMENTS.DOCUMENT holds 2 xml file after doing the payment. One xml file holds many information. Another one has only those xml tags which gets generated through ISO20022CGI.xsl.