OSGi jar which are known as bundle requires OSGi metadata in its manifest so that an OSGi framework can manage the visibility of classes between bundles. A JAR without this metadata would only contain invisible classes, could not see any classes from other bundles, nor could it get started in any way. The Import-Package manifest header tells what packages should be made visible to the bundle, and the Export-Package defines the packages in the bundle that should be made visible to others.
Normal jar wouldnt have this metadata or a manifest file