Does anyone know what these rows are for?
Is it safe to remove them? Since they seem to be mostly empty?
Note: this is assets table!
@Salivan, I see why you want to remove those assets now. Just curious: how did you remove the articles and tags? via database or within Joomla?
Now to remove the assets, you should make sure that these are orphaned ones and are not relating to current data.
If you have removed everything then you could clear these assets as well - but first backup the table in case something breaks.
If those assets are related to articles that are still in the db, then you shouldn't delete them. If you try to do this manually then there will be work, as you will have to deal with finding tags and articles that are still there.
You might have some better luck by using the ACL_MANAGER component, which deals with fixing assets - it might be able to remove those orphaned assets.
You can ask the developer if the extension is able to remove those particular orphaned assets, before purchase.
Actually, I did a quick test with aclmanager to see if it deals with the assets for content-items and tags: added a tag to an article, then deleted the tag and the article within Joomla admin, run the acl_manager extension, but the corresponding assets is still in the db.
So I guess some kind of "manual" approach should be followed in order to clear those assets.
I have some better news, regarding the assets. They are able to be recreated if they do not exist, for any remaining articles/tags. So, if you delete all the assets of type #__ucm_content.xxx, even for articles/tags that exist, they can be recreated by doing re-saving the articles - or doing some kind of batch changing (.e.g change category for many articles).
So, I would suggest, in a test environment, you can do your trials with these and work on the database to see how things go.
#__ucm_content database table is used to store tags for core extensions, so I would not remove them, else you may get an error along the line in the future.
See more information tags with extensions here:
hope this helps