I do not completely understand your questions but I will try to do my best. Otherwise, please rephrase your questions.
I presume you have access to the LSA training system as a PVS (Provate Virtual Server) as this it a Pega Academy forum.
A declare page, or a data page, can be set up as a list and/or as a look-up. The latter having a parameter to find the correct record. You do this on the Parameter Tab on the Data Page rule. If your datapage uses a report to load the data you need to set up a parameter on the report and pass it on on the datapage deinition tab (See the Data Page Position on the Cadidate caste type.
Now you can create a page property on the candidate. If you configure this property to refer (or copy, depending how you going to use it) to a datapage this page property now displays a texbox where you can enter the key to the data. This can be another property on your work item (See the Position Property and PositionApliedFor property on the Candidate case)
Another posibillity is to have a section showing data from a declare page. It can be configured, just like a page property, to use a data page for its content and you supply a parameter, mostly a property from your work item (See Select NewPosition section on the Candidate Case)
Hope this gives you some hints. If not please post again.