Almost all page layout scenarios can be accomplished using from 1 to 3 column layout but occasionally you run into a situation where you would like to be able to edit content above or below these 3 columns. To accomodate this there is support for 2 optional extra content placeholders in the layout.master file of the skin. The skin named ramblingsoul-5contentpanes (found in the extra-skins.zip download) illustrates this feature by putting one above and one below the 3 center columns.
<asp:Panel ID="divAlt1" runat="server" CssClass="altcontent1">
<asp:ContentPlaceHolder ID="altContent1" runat="server"></asp:ContentPlaceHolder>
<asp:Panel ID="divAltContent2" runat="server" CssClass="altcontent2">
<asp:ContentPlaceHolder ID="altContent2" runat="server"></asp:ContentPlaceHolder>
If these are present in the layout.master they will show up in the PageLayout page and in the Content Manager Publishing page as options and you can place content instances into them. If you later change to a skin that does not have these optional content panes the content will be rendered in the center.
Note this feature is for advanced scenarios, its really not recommended to use these unless you have a specific need for them. It doesn't make sense for example to put a blog in the header or footer and this is true of other features too. The bottom line is if it makes sense for what you are trying to do then use it but don't post in the forums asking why it doesn't look right if you put your blog in the header.
Note that if these panels don't have content instances in them, they will be hidden by default. If you want to always show them even when they have no content instances in them, you can set these properties on StyleSheetCombiner control in your layout.master file:
<portal:StyleSheetCombiner HideEmptyAlt1="false" HideEmptyAlt2="false"...