However I think there is difference between tag and categories, can you create nested tags?! , but you can categorize the questions by parent and or child categories.

please let I want to create a forum for a big portal with hundred subject, for example: poem, mathematics, programming, astronomy, physics, and many other categories,
with current condition i have to solution:
1-categorize this subjects with tags (so i miss the child subjects like front-end and back-end developing in programming)
2- use an instance of Live Exchange for each above subjects. that mange of too many instance looks terrible!

so i think it's better to add category to Live Exchange and Live Forum
Regards

In 'filter by tags' you can enter several tags so you can search for one or more tags at the same time. I would not use many instances of Live Forums or Live Exchange.

Nested categories are great if you offer navigation / visualization. However, latest trend has been to move away from navigation to search based solutions. It's much easier to search for a topic then to navigate through a set of nested categories.

OK,
I was convinced :)

I almost think both are needed.  two ways to find what you need is better than one.

Also it would be nice to be able to restrict categories by role so you can make private and group forums.

I am also looking for categories for Live Forums. I have been doing lots of research on this and found a great article about the difference between Categories and Tags. I think having categories as well as tags would be very helpful and help organize and streamline Live Forums (especially for large groups with heavy forum use). The article talks about blogs but I think it is applies to forums posts as well.

https://www.wpbeginner.com/beginners-guide/categories-vs-tags-seo-best-practices-which-one-is-better/

Pulling an excerpt from the article:

Categories are meant for broad grouping of your posts. Think of these as general topics or the table of contents for your site. Categories are there to help identify what your blog is really about. It is to assist readers finding the right type of content on your site. Categories are hierarchical, so you can sub-categories.

Tags are meant to describe specific details of your posts. Think of these as your site’s index words. They are the micro-data that you can use to micro-categorize your content. Tags are not hierarchical.

For example if you have a personal blog where you write about your life. Your categories can be something like: Music, Food, Travel, Rambling, and Books. Now when you write a post about something that you ate, you will add it in the Food category. You can add tags like pizza, pasta, steak etc.