A page is based on a layout. A layout contains placeholders and in a page based on that layout those placeholders will become drop zones where you can insert widgets.
One of the placeholders is special and has 'Content' as its type. If you open a page in a popup only the widgets will be shown that are placed in the area that corresponds with the Content placeholder.
Usability of these new features can and will be improved during this beta stage. Thank you for sharing your experiences with the beta.
Note: beta6 has been released and it fixes problems with importing modules into the Modeler. We will keep on releasing beta versions based on your feedback.
Rand - I'm not a 5.0 expert (I've just seen it for the first time today), but it seems to me that the layout objects are use to set up page styles. One of the object types you can put into a layout object is a placeholder. If you look at the properties of a placeholder object, there are two types: Normal and Content. Content is, I think, the place on a web page where data views, template grids, widgets, etc will go when a layout is used for a particular page. And, as the CSS structure likely allows for one Content object only, Mendix will only allow you to place one placeholder with Type Content in each layout object.
Let me know if that helps......Mike