In the KT it's possible to create new KT templates with some TAGs (TAG_SUMMARY, TAG_PROBLEM, etc) in it. But what are TAG_CUSTOM_1, TAG_CUSTOM_2, etc. used for? I cannot see any entryfield where I can put some text in it?
The TAG_CUSTOM_ fields are attributes, that are added to the database schema only. They cannot be used out-of-the-box.
If you want to use them, you need to customize the Knowledge Document Detail screen (detail_KD.htmpl form) with the WebScreenPainter and add textboxes with the attribute relating back to the custom* fields.