During the search post-process, the plugin parses the Post/Page/CPT titles and the contents, as defined by these fields. First it looks for the Primary field, and if it is not found or empty, it uses the Secondary field.
It is possible to define a single, text based custom field as the title or description field.
The Yoast SEO plugin uses the folling custom fields to store the custom title and meta desctiption fields:
The All in One SEO plugin uses the following custom fields: