LXFML is an extension of files specific to the program that allows you to design 3D models from LEGO bricks called LEGO Digital Designer. LXMFL is a file used to store projects made in the program. Technically speaking, it is a text file containing XML code. It defines places where individual elements (blocks) of the project are located.

LXFML is part of the archive saved in the LXF file, which also includes files with the MODEL and PNG extensions.