Tixati has nice feature - Transfer options for specific category (Right click on category name in categories sidebar).

It'll be nice to have subcategories in tixati.
For example:
Category Video, Subcategories Movies, TV.
Category Tracker Public, Subcategories: Movies, Books, Apps
Category Tracker Private, Subcategories: Movies, Books, Apps

For example seeding options can be set as is now for category, but categories could help to organize torrents inside main category and on disk (specified download locations for each subcategory).

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In tixati 2.16 there is something wrong in "Load transfer" window and category submenu.
I set 1 separator @@@, and 2 categories: cat1, cat2.
And in category submenu in this window I see something like this:
Uncategorized
@@@ (I have click here to see cat1, cat2)
  cat1
  cat2

In tixati 2.12 there is:
Uncategorized
cat1
cat2

I don't see any point having separator in there. If there'd be subcategories for example it'll be nice.
Imagine starting new transfer, changing category to public/movies and movie is saved in C:/movies, public/apps - C:/apps, private/movies - C:/movies.
Now when downloading movie I have to set category to public/private tracker to have specified bandwidth settings for each category, then in "move on complete" change manually download location to C:/movies.

you can kinda do that. make a separator as video and then under it have categories for movies, and tv.

as for the categories menu on load transfer prompt, that is how it is supposed to work. cat 1 and cat 2 are supposed to be seen over from the separator.

But this doesn't work with bandwidth settings for each category.
For example I want only 2 categories with subcategories in them.
Category "public tracker", incoming speed set to 500KB/s, outgoing 300KB/s with subcategories (movies - C:/movies, apps - C:/appsPub).
Same for "private tracker", incoming/outgoing speed no limit, have subcat (movies - C:/movies, apps - C:/appsPriv).

With this approach 500KB/s incoming speed for "public tracker" is in total for all subcategories.

Of course I can create:
Separator "public tracker", category pub-movies, category pub-apps, ....
Separator "private tracker", category priv-movies, category priv-apps, ....
But what speed limits for each category under separator "public tracker" do I have to set if I want 500KB/s total (pub-movies + pub-apps + ... = 500KB/s)?

With your suggestion achieving this is not possible.

Please how to make that "separator"?

I am also missing subcategories.

I have some already existing structure of files on my HDD and Tixati breaks it by not supporting adding subcategories. Subcategories should be under main categories and collapsible so it do not take too much space in categories list.

On Categories sidebar is the plus button, open Add Category window with it and choose Separator there. If I understand it correctly it is only graphical separator, it won't work as higher level Category.