Use this tag—which is very similar to JSP's jsp:include—to encapsulate and reuse content among multiple XHTML pages. There are three things this tag can include: plain XHTML, and XHTML pages that have either a composition tag or a component tag.
You supply a filename, through ui:include's src attribute for JSF to include. That filename is relative to the XHTML file that was rendered as a result of the last request. So, for example, if JSF loaded the view login.xhtml, and that file included pageDecorations/header.xhtml, and pageDecorations/header.xhtml included companyLogo.xhtml, then companyLogo.xhtml will not be found if it's in the pageDecorations directory, because companyLogo.xhtml has to be in the same directory as login.xhtml.
javax.el.ValueExpression (must evaluate to java.lang.String)
The filename of an XHTML page to include. The filename is relative to the XHTML page that was originally loaded.