5 things about PeopleSoft Page Composer

1. Page Composer creates Fluid pages through configuration.  At the moment this is only available for Fluid Approvals (introduced in HCM & FSCM Images 020 and CRM 012 and ELM 015) but as a general framework for page construction you can see where Oracle is taking this.  It is part of Enterprise Components NOT PeopleTools. Although you will need to be on at least PeopleTools 8.54.27 or 8.55.10 (See Doc ID 2166359.1) 
   
   
   
   
2. Page Composer pages are based on a Page Composer Registration in which you define the sections of the page (summary, header, detail, etc) and the data sources and custom business logic.  You can define up to 5 header and 5 line sections.  
3. There is currently only one Page Composer composition type of APPROVALS.  The approvals pages are all based on the same component EOAWMA_MAIN_FL.  Page definition EOAWMA_RSLT_SBF renders the initial summary list view and page definitions EOAWMA_TXNHDTL_FL (header) and EOAWMA_TXNLDTL_FL (line) render the Detail you see when you click one of the summary list items.  (TOP TIP:  Of all the pages delivered with PeopleSoft I think I would try and avoid customising this page due to a) it's high complexity and b) the likelihood of it changing with each image release.  
   
4. When defining the Page Composer Registration it takes a bit of trial and error to identify which page fields can be referenced and used.  Suggest looking at delivered Page Composer definitions in Image 020 and cribbing these for your own definitions.  
5. You can assign custom PeopleCode to page field events.  Create an App Package class which extends class EOPC_RUNTIME:pcFieldEventAction and assign to a field in the Page Composer Registration Field Event page.