Welcome to the Q&A Forum

Thank you Chris

Hello,

I have some awareness of how to use H1, H2 and H3.
H1 only once per page as the main page heading.
H2's should be subheadings, H3's are sub-sub headings of the and so on. 
This structure gives hierarchy and opportunities to use additional keywords in an order of priority.

I can clearly understand how this would work in an article but what about other content on the page such as global/frequently repeated elements like sidebars and footers?

I see sites - and in particular, I have examed SEO focused sites - that use  H3, H4 and H5 in these instances seemingly giving themselves scope to use at least H2 tags as part of the page content and break out of the structure hierarchy when dealing with sidebars and footers.

I suppose this could signal theses headings are sections of the page that are less relevant than the main article content but that is just an assumption. I don't know what is correct.