What does the REL tag mean in HTML?

The rel attribute of the <a> label is used to specify the relationship between the current document and the linked document.
Optional properties for <a> tags rel and rev represent the formal relationship and direction between the source document and the target document, respectively. The rel attribute specifies
The relationship from the source document to the target document,
The Rev property specifies the relationship from the target document to the source document. Both of these properties can be used simultaneously in the <a> tab.
The value of the Rel or Rev property is a space-delimited list of relationships. The actual relationship names and their meanings depend on yourself: the HTML or XHTML standard and
Neither of these attributes has been formally proposed.
For example, a document in a series of documents might include its relationship in a link:

What does the REL tag mean in HTML?