Advanced Categories
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Why would I need Advanced Categories?

Just like a lot of add-ons Advanced Categories is not required for every site, but some sites need more control over how categories are assigned to entries, and ExpressionEngine's native category fields do not have any rules around category selection. Also, as your site ages, its possible that some categories become deprecated, or category usage changes.
For example, maybe you don't want any new entries to be assigned to certain categories or category groups, but have hundreds or thousands of old entries where those category assignments are still relevant.
Another example, maybe your category groups make sense and are still used to display content on the front-end, but you only want the user to be able to assign a single category from one of two or more category groups. Advanced Categories lets you create a custom field with a single list of categories from multiple groups, and limit it to a single category selection.
With Advanced Categories you can customize which categories, from any group, are presented to an user when editing an entry. There are a lot of different reasons someone may want to drastically alter how categories are managed on an entry. Advanced Categories is designed to fit those needs.

Does Advanced Categories replace the category groups on the entry publish page?

No, it does not replace the the Categories tab or the category group fields that appear on you rentry publish pages. Advanced Categories is a separate, standalone fieldtype that acts as a facade for assigning categories to an entry.

What happens if I uninstall Advanced Categories?

Since Advanced Categories is just a custom fieldtype that acts as a facade for saving categories to an entry, uninstalling the add-on will not remove any assigned categories to any entries. Uninstalling it will simply revert the entry publish form to the state it was in before you installed it. If you're using the Advanced Categories custom fields in your front-end templates, such as {primary_category:ids} then obviously you'll need to remove the field tags from your templates.

Wouldn't adding separate category fields to an entry publish page confuse editors?

Well, this is always a possiblity, but this is why it is strongly suggested to use the Publish Layouts feature in ExpressionEngine to either hide the default Categories tab entirely, or certain category groups. You can do this globally, or by user group, depending on what level of experience or knowledge your users have with the system. For Super Admin users, or highly experienced users maybe you want to keep the default Categories tab and category groups visible. For less experienced, or trusted users, hiding the default Categories tab and or category groups may be a better apporach.

Is there a template tag to list categories?

If this is a question asking about a module template tag to replace the {exp:channel:categories} tag, then no. This is fieldtype only to assign categories to an entry. It is not, and will not, include any functionality to list categories outside of the context of a custom field.

How do I display nested categories?

You can't, and there are no plans to support it. Since Advanced Categories lets you customize which categories are displayed, which means you can create more than 1 Advanced Categories field to display child relationships. Advanced Categories is intended to help simplify category display and selection. It is of our opinion that nested categories only complicate matters. Simple, flat category listing or bust :)