Recently USFDA released an updated "Electronic Common Technical Document (eCTD) Technical Conformance Guide" that revised some sections based on ICH eCTD v4.0 Implementation package including removal of Transition Mapping Message and addition of Forward Compatibility.
The purpose of this Technical Conformance Guide is to provide specifications, recommendations, and general considerations for electronic submissions based on eCTD to the Center for Drug Evaluation and Research (CDER) or the Center for Biologics Evaluation and Research (CBER).
Sponsors and applicants can use this guide to get technical recommendations for the standardized electronic submission format for INDs, NDAs, ANDAs, BLAs, and master files. It is designed to enhance and promote collaboration between sponsors, applicants, and FDA's electronic submission support staff.
Forward Compatibility is used for any dossier that has v3.2.2 content and the application is being converted to a v4.0 message.
Click on this LINK to know more about the general guidelines and considerations, submission contents details etc.
Also, click the below links respectively to check on the updated information on eCTD 4.0