1. What is termed as a deployable object? Is it an archive i.e. a WAR, EAR, etc. Or something that comes to being after deployment of an archive?
Yes it is archive file which you deploy.
2. Can I create my own archive type?
No. It must be recognizable by the app server.
3. Can I access some resource of another deployed web archive? e.g. f I have an XML descriptor in another WAR deployed on the server, how can I access it?
Why would you need to access deployment descriptor of another WAR file.The DD is for server to understand. If you are asking if you access file in an external jar you may have to use
java JAR apis.
4. Would all this be application server specific or could it be made portable?
Each deployable jar file has app server specific XML.