One of important aspects of search engines optimization is document structure. Text data of HTML page consists of headers of different level (there 6 header levels in HTML) and from the plain text content. To create good document structure you should use #h1, #h2, #h3, #h4, #h5, #h6 tags. It is quite obvious that a header of the corresponding level (1, 2, 3, 4, 5 or 6) will be created in each case. The lower the header level is the more important it is. Headers on the page should go from the lowest to the highest header level.
Automatic PSD to HTML conversion service is will create HTML header of the desired priority level if you mark a Photoshop text layer with the corresponding tag.
More on using HTML headers you will find on the documentation page of the PSD to CSS service.