You have a new project and legacy translation memory files that usually go to /tm folder of OmegaT project. You need to give this project to someone else, but you don’t want to give away all of your previous translation. Somehow you need to extract from your TMX files only those TU’s that have matches in the current project.
The problem is evident — you need OmegaT to get matches for each segment, and if they are any good, store them somewhere handy, in a separate TMX file.
The problem has been (or still is) discussed on OmegaT Yahoo Group.