CTX_DOC.HIGHLIGHT and CTX_DOC.POLICY_HIGHLIGHT take a query and a document and return offset information for the query in either plaintext or HTML formats. This offset information can be used to write your own custom routines for displaying documents.
CTX_DOC.HIGHLIGHT and CTX_DOC.POLICY_HIGHLIGHT are equivalent, except that CTX_DOC.POLICY_HIGHLIGHT does not require an index.
With offset information, you can display a highlighted version of document as desired. For example, you can display the document with different font types or colors rather than using the standard plain text markup obtained from CTX_DOC.MARKUP.
Oracle Text Reference for more information about using CTX_DOC.HIGHLIGHT and CTX_DOC.POLICY_HIGHLIGHT