Question: I am writing a paper and I am lost for a phrase to describe the set of topics a person is knowledgeable about.

For example: "If someone knows about topic A and also knows about topic B, then topics A and B are [phrase relating the topics and the user's state of knowing]."

Answer: 
Those topics can be said to be “*in his **wheelhouse***”

Defined in [Dictionary.com](https://www.dictionary.com/browse/wheelhouse) as:

> 
> within one’s area of expertise or interest:  
> 
> *There are some subjects that are in your wheelhouse and some that are not.*
> 
> 
>
The answer to your question will depend on the precise area(s) of knowledge you're referring to, the level or degree of expertise of the knower, and the intended register.

One option not previously mentioned is the word "bailiwick," which is fairly productive, as it can be used both phrasally and as a stand-alone noun.

*You should ask Laura that question: It's in more in her bailiwick than mine.*
*That question falls outside my bailiwick.*

[Merriam-Webster](https://www.merriam-webster.com/dictionary/bailiwick#examples) gives these example sentences:

*Questions about organization of the fund drive are my bailiwick.*

*Weaver’s bailiwick has been Republican candidates whose greatest interest seems to be criticizing other Republicans.*

*Reardon's usual focus is on using data from pulsar timing arrays to search for nanohertz-frequency gravitational waves, so magnets are a bit out of his bailiwick.*

A few alternatives (again, depending on context) involve the words "scope," "competence," and "expertise."