We recommend that you resort to this option only when you want to seek information external to the text of the letters, i.e., information that is impossible to detect by the Advanced Search.
Here, in Raw Search, the system will identify individual words, regardless of their location in the XML file. You may find uninteresting things, like all names of XML-TEI elements and attributes (e.g. unclear, id). But you may also find uncommon words, occurring in the title, in the abstract or in the context of the edited document (e.g. embarcadiço).
Raw Search enables you to find any of the words either in the header or in the body of the electronic document, but for those users who specifically seek epistolary text, the Advanced Search is more suitable.