This facet can be added dynamically to a document to store arbitrary related text that will be used in the fulltext index for the document. In this way, you can find the document with a fulltext search on more that just what's stored in the document.
This is used for instance to add the Annotations fulltext to the document, or in the Semantic Entities plugin. We'd like to do it for comments as well but it's not done yet.
It can be used by third-parties to add further text.
Of course this requires that the fulltext index includes the
relatedtext schema. This is the default, but if you redefine the default index it's important to know.