Have a look at the Placeholder Tags List in the Help Menu. It contains the following text about headers and footers…
Headers and Footers
Not all tags are supported in headers and footers. In addition to the special header and footer tags specified below (which can only be used in headers and footers), only the tags listed under the sections Page Numbers, Current Date and Time, and User and Project Information are supported in headers and footers.
<$sectiontitle>
Gets replaced with the title of the document that first comes after the page break most recently preceding the header or footer in which it occurs. In practice, this is useful for placing chapter titles in headers or footers. For instance, if you have a chapter in a single text document starting on a new page, then that document’s title will replace “<$sectiontitle>”; if your chapters are broken down into smaller sections placed inside folders, and the folders mark the start of each new chapter, then the folder’s title will replace “<$sectiontitle>” throughout the chapter headers and footers.
Note that if the section title cannot be calculated for any reason (for instance, because there is no text in the section associated with any documents), <$sectiontitle> may fall back on using the abbrevated project title (working the same as “<$abbr_projecttitle>” - see User and Project Information for details on where the abbreviated project title is drawn from).
I’m not sure that you can do this just for selected documents though - I think it’s a Project wide setting.
You can auto-generate synopses for selected documents by selecting them in the binder and choosing Documents > Auto-generate Synopses. The synopsis is taken from the first 500 characters of the text.
HTH
David