@Lluis Machado Azure form recognizer does not provide a validation method to classify documents based on the content in the document.
Usually, customers using the service develop automation around the file names and place the files in different folders or containers before passing it to the corresponding prebuilt model for analysis.
Another method that is used is by uploading documents from a client-side app or website against the corresponding label or upload placeholder and the application takes care of moving it to corresponding folder on storage for analysis by the pre-built model.
I hope this helps!!
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