The aside element represents content that is tangentially related to the main text of a document. It aligns with what we think of as a sidebar.
As with the other structural tags its not about position within a document, but rather its relationship to the content. An aside doesn’t have to be to the right or left as we typically think of a sidebar. It could be at the top, the bottom, or even in the middle of a section of content.
If the information could be removed without reducing the meaning of the overall content, the information is a good candidate for an aside. Pull quotes are good examples. Advertising another.