Organization should be built once and then never imported again...it should be maintained online after that. Or exported and imported during out of hours. importing an organization overwrites a previously existing organization.
Organization can be built in Studio, or if you are adventurous by creating a XML file following the rules of the original XML. Export from Studio, open review and create...I did it from excel a while ago.
API extensions you only need to do this if you've created any, if you're asking this question, you haven't, so no need to complicate matters :)
Pages these are your custom designed application pages (not part of a process) that may* have been developed in the UI, you Export them from the UI and then import them via Admin->Resources.
may* = it is possible to create custom pages in a third party piece of software and then craft the zip file from this. This can then be imported accordingly.
layouts if you are using community this is not relevant as you don't get to customize them.
themes if you are using community this is not relevant as you don't get to customize them.
profiles if you are using community this is not relevant as you don't get to customize them.
Business Data Model these should be imported only if they are presented outside a process as a process export should bring them anyway.
PS: If this reply answers your question, please mark a resolved.
Thank you Sean I'm testing what you said and will inform you of results .. ;))
Thank you again Sean
I exported and imported a page successfully and you were right.
Still application does not get completed and it stops after the first page in the first form.
I fill the form fields and click on submit button but process stops and doesn't get completed. This is whilst I can run and complete the process in studio and then review in portal and approve or refuse it.
Meanwhile the application profile is user, but I should enter in the administrator mode so that I can see the list of applications. How can a user execute an application?