Custom page layouts can be handled in two ways:
Reality is that most portals use custom page layouts (and custom master pages) and therefor there's a need for page layout mappings for those custom page layouts. The publishing page transformation component will have a 'default' page layout mapping for all the out of the box page layouts, so if your portal is using those out of the box page layouts you're covered. When transforming publishing pages it's therefor mandatory to map the used page layouts into a publishing page transformation model. Those two pages combined with fields containing data make up the page a user sees in the browser. Publishing pages are always based upon a page layout and a master page.
