Imagine there are 2 domains.
Users on 1st domain are requesting data from 2nd domain, and thus retrieving data from there, or writing new data back to it to it or update. Although this is CRUD, but works across domains. Cross-domain CRUD operations, to involve jsonp, because ajax is limited to Same-Domain policy only.
2 more concise requirements.
Users to first login across domains, before they can pull data from the 2nd one via web-servicing. So implement cross-domains or across-domains login and logout.
Users to do session authentication across domains, so that users no need to login everytime they want access a different page on the data-processing domain.
Lastly, Html5 pages only on first domain , requesting data from the 2nd domain, via jsonp.