You can include a main category in the root element, as well as sub category elements to group different sub-sets within a single TriviaML file. Categories inherit the already named category of the root element, or parent categories. A category can also have a title attribute with a free string. Categories can be infinitely nested.
Example

<triviaml category="science">

    <category name="chemistry">
        <trivia>
            ...
        </trivia>
        <trivia>
            ...
        </trivia>
    </category>

    <category name="psychology">
        <trivia>
            ...
        </trivia>
    </category>

    <trivia>
        ...
    </trivia>

</triviaml>

The following category structure is suggested, but you can freely expand on it (feedback welcome):

+ science
    + math
        + calculation
    + chemistry
    + psychology
    + biology
        + animals
    + physics
    + astronomy
    + medicine
    + computer
        + hardware
        + programming
        + internet
        + people
        + history
+ sports
    + racing
        + car
    + tennis
    + basketball
    + baseball
    + football
        + super bowl
    + olympics
        + winter
        + summer
    + records
    + boxing
+ entertainment
    + quotes
    + people
    + movies
        + stars
        + films
        + awards
            + oscars
        + quotes
        + genre:
            + horror
            + animation
            + comedy
            + musical
            + science fiction
            + fantasy
            + sport
            + drama
            + action
            + war
        + decade:
            + 1920
            + 1930
            + 1940
            + 1950
            + 1960
            + 1970
            + 1980
            + 1990
            + 2000
    + tv
        + series
            + seinfeld
            + friends
            + er
            + star trek
                + original
                + next generation
                + *
            + *
+ history
    + people
    + events
    + inventions
        + 14th century
        + 16th century
        + 17th century
        + 18th century
        + 19th century
        + 20th century
        + 21st century
        + computer
    + politics
        + cold war
    * wars
        + first world war
        + second world war
+ arts
    + paintings
    + design
    + architecture
+ geography
    + capitals~
    + mountains~
    + rivers~
    + cities~
    + africa
    + europe
        + uk
        + germany
        + italy
        + spain
    + asia
    + australia
    + america
        + usa
    + culture
+ music
    + lyrics
    + stars
    + songs
+ literature
    + quotes
    + poetry
    + authors
        + umberto eco
        + douglas adams
        + jack kerouac
        + *
    + comics
        + superheroes
            + golden age
            + silver age
            + marvel
                + spider-man
                + fantastic four
                + hulk
                + *
            + dc
                + batman
                + superman
                + *
            + image
        + alternative
        + people
+ puzzle
    + true or false
    + trick questions
    + math
+ misc
    + *

Example: category="comics/superheroes/marvel/spiderman".

Legend:
+ Category
* Additions especially welcome
~ Inherits to sub-categories