There is only one sidebar available, on the left-hand side. If no blocks are assigned to the sidebar, the page will render as full-width.
The sidebar may contain any block type (text block, menu block, view block, iconoblock, pictowide, etc.), but there are three blocks that exist only in the sidebar. They should appear in the order shown below.
The sidebar menu is for contextual links, or, links directly related to the content of the page. These are menu blocks, with the CSS class default-menu.
The search box should be available on every page. This will search the content of your site only.
The Related Links box is for external links to further resources. It is a standard menu block with the CSS class related-menu.