InDesign Newbie Margin Woes

[vandala]

Hi,

Please excuse my ignorance here; I'm new to InDesign, and while I'm becoming relatively proficient with typesetting and layout, I appear to be failing to grasp one or two fundamental concepts, which is playing havoc with my margin layout.

When one sets margins on InDesign, this creates a "a type area" and "folio" areas, correct? The type area becomes the place for body text, graphics, headlines, etc, while the folios (outside the body area) are the places to stick nice banners and page numbers and the like, right?

Now, all this seems correct in theory. My margin setttings are pretty straightforward (3p5 top, 3p0 bottom, 2p5 inside, 3p0 outside). It's an A4 page. However, when I place items outside the type area, they are obscured in the print out and don't show up at all. I thought this was a printer problem at first, but if I create a PDF of the page, the same thing happens.

However, the printer menu (in InDesign) has some kind of "offset" option for printer's marks, etc, which seems to be setting space all around the document. The default is set to 0p6. When I set this to 0, it seems to create the PDF with the right dimensions, presumably because it's no longer trying to allocate space for the printer's marks on an already tight a4 page.

So, my question/s are...

1) Is the logic above correct? Obviously it makes sense to set up a page that allows the placing of objects outside of the type area, so why does InDesign not want to do it?
2) Is the problem related to this "Offset" option in the print menu? If I turn it off, I guess it's leaving no room for printer's marks. Is this going to present a problem for a commecial printer?
3) Or have I got the wrong end of the stick entirely?!

Apologies for the rambling...