Custom Tags are tags you define yourself, based on your own criteria or logic. Rather than Segwise's AI determining what to tag, you tell Segwise what to look for. This is in contrast to the default tags, which are fully automated.
Note: Custom Tags in Segwise are created and configured through a separate application called Segwise Tag Bot, which you access via Settings → AI Tags → Create Custom Tags
Screenshot: The Custom Tags entry point under Settings → AI Tags, which opens the Segwise Tag Bot.