When a JAR is signed, parameters in JAD file and in JAR manifest must match, or else the application will not be installed. If the application is not signed, they do not need to match and JAD properties have precedence over Manifest.mf’s.
When a JAR is signed, an installation is only successful when all the verifications are successful (e.g. digital signing, JAD parameters, etc)
A signed application may never be updated with an unsigned application
You can define the required permissions on the JAD file:
MIDlet – Permission: Required permissions for this suite. If they are not available, the suite will not be installed.
MIDlet- Permission- Opt: Permissions that the device may use, but might work without, even if in a limited way (e.g. game may have multiplayer support, but may work without network access in a singleplayer mode)