How Does the Related Documents Plugin Determine the Suggested Related Documents

When using the Related Documents plugin on a document type, there are a list of Related documents suggestions. On what parameters does the plugin determine which documents are relevant?

There are two ways,
SimilaritySearchRelatedDocsProvider
Searches for Nodes (documents) that are related based on Lucene's rep:similar
ReferringRelatedDocsProvider
Which seems to return documents that reference the current one.