Note: You are currently viewing documentation for Moodle 3.1. Up-to-date documentation for the latest stable version of Moodle is probably available here: Using tags.
The related tags that appear in a tag page comes from two sources:
1) User suggested related tags, entered in the "Related tags" field.
2) System suggested related tags, which are automatically calculated. Tags that appear a lot together ("Computers" and "Software" for example) will be considered related.