If your HTML block begins with "(optional space)" followed by "(any one optional tag)" followed by "(optional space)" followed by "<!DOCTYPE " then your HTML block will be used for the entire HTML response, and TheImageFile content management system will not provide any title or headers or anything else, and will ignore the two Rich Text areas. Use this format is you want to control your own <head> section or if you are creating a frameset.
If your HTML block begins with "<body>" then your HTML will be used for the entire "<body>" of the response, and consequently any headers, footers, side bars, or nav menu will be automatically suppressed. The two Rich Text areas will also be suppressed. Use this for fullscreen authoring. TheImageFile content management system will still manage the <head> section including title and meta tags.
Otherwise, if your HTML begins with anything else, then your HTML is considered a rectangle that will appear in the middle of the page, surrounded by the standard header, footer, sidebar, and side-image elements. Any upper Rich Text will appear at the top of your content rectangle, and any lower Rich Text will appear at the bottom.
