Not Considering

TM fields are in project settings domain, they should not be stored in the SDL XLIFF files.

Save all custom fields information in the SDLXLIFF files

Today, the System fields are in the SDLXLIFF, but not the custom fields, which means that if I  rebuild a TM from SDLXLIFF files, I lose all the custom fields information.  I would propose to keep this information in both the TM and the SDLXLIFF files.