You must read the section Distributable Java programs: jar files in the lesson 25 notes.

The name "Manifest.txt" means ... nothing

The name "Manifest.txt" actually has no special meaning. You could call it "George" and everything would work out fine, as long as it's in the right place on the "jar" command's arguments. So you could have files ManifestA.txt and ManifestB.txt that refer to different "Main-Class"s, like this

Main-Class: Dog
Main-Class: Cat

and give the commands
jar cfm Foo.jar ManifestA.txt *.class
jar cfm Bar.jar ManifestB.txt *.class
and everything would be fine. The command
java -jar Foo.jar
would run the program starting with Dog's "main" method, and the command
java -jar Bar.jar
would run the program starting with Cat's "main" method. So in the context of this project, you can have a manifest for making the Part 6 jar file, and another one for making the Extra Credit jar file, and they can both sit in the same directory and not interfere with one another.

Emailing jar files

As discussed in the lecture notes, you have to be really carefull when it comes to running other people's jar files - you need to be really sure you know what the source of the .jar file is and that you trust that person. That being said, if you want to e-mail your .jar file to someone ... you can't. Gmail will block your e-mail if it has a .jar file attachment. You should already know the workaround here: rename the file (e.g. instead of Foo.jar, rename it Foo.raj) and attach the renamed file to your e-mail. Of course this means the person on the other end will have to change the name back. After that, though, they can run it for themselves (e.g. on a Windows machine by double clicking it).