File Extension SJX File Format Seljax Estimating and 3D Design SJX stands for Seljax Estimating and 3D Design. This was last updated in March 2012 By Margaret Rouse Browse File Extensions Alphabetically: A B C D E F G H I J K L M N O P Q R S T U V W X Y Z # Ask a Question. Find an Answer. Ask your peers a question about this file extension. Ask a Question Estimating offline in SAP Estimation Template for Testing Team Do you have a favorite construction estimating software tool?