MXLIFF files are Memsource specific files based on XLIFF 1.2. The core of the file is XML and is designed for localization tools. They hold both source and target information when exported from a job.
MXLIFF documents are primarily intended for translation inside Memsource Editor or to be imported into Memsource directly via the UI and can be downloaded from the editor from the menu if download access of MXLIFF files is not restricted by a project manager.
Once finished processing in Desktop Editor or other tools, MXLIFF files can be imported via Tools > Upload.... Multiple or large MXLIFF files may be imported in a single ZIP file for convenience and a quicker upload.
When translating MXLIFF documents outside of Memsource, it cannot be guaranteed how the files will be processed by other software.
MXLIFF files are not automatically recognized by SDL Trados and are only imported as reference documents. To import an MXLIFF file into SDL Trados as a translatable file, the extension must be changed to XLIFF.
SDL Trados adds additional information to XLIFF documents that are not a native part of an MXLIFF document. Select the Preserve; don't add if not originally present BOM option and the Do not store segmentation information in the translated file setting when exporting from SDL Trados to produce a file that be used with Memsource.
See Marek Pawelec's blog post for current instructions.