Recently I tried creating an Adaptive form using xdp as a template in AemAACS, but I don't see the option of uploaded xdp file in model while creating the form, if xdp files are not used for creating forms anymore what are the xdp files or designer forms used for?
I think Forms CS does not support creation of an Adaptive Form compliant to the XFA form. However, you can create an Adaptive Form out of a given XFA form by using the Automated Form Conversion Service. XFA form is also used as a Document of Record bound to the given Adaptive Form such that if you have to send a regulatory or otherwise PDF document to the person filling the form, you could get this PDF document easily out of the Adaptive Form data submission.
With the XDP file, you define the form template, that is, the design of the form which is based on the XML schema. You create the XML schema and the Adobe XDP file in the SAP NetWeaver Developer Studio in a dedicated development component (DC) called Enterprise Content Management (ECM).