there are two distribution methods: you can either use webstart, or you can use a zip/jar archive, or you can use both.
if you choose to use webstart, everything is fine, and you don't need a manifest thanks to the JNLP spec.
if you choose to use a zip file, the included java program must be able to executable without a command line (i.e. no batch files). in most cases zip files are used for applets.
if you choose to use a standalone jar, it must be self executable with no command line or batch scripts or anything OS-specific like that.
sometimes you have to use two different jars if you want to distribute with webstart AND a jar file. for example, a non-manifested webstart jar might use up that free space with signing, whereas the standalone jar (which never needs to be signed) might use the extra space for the manifest.
note that when I say that a jar must be self executable, I mean it must be self executable on an OS that is properly configured to run a jar file. That is, double-clicking and that sort of thing. If you have a weird setup, that's your problem, not the game authors