Xray provides a set of custom fields that can be used for some issue types as detailed in Custom Fields and Screen Configuration.

Besides this, you can add your own custom fields to any Xray issue type: Tests, Pre-Conditions, Test Sets, Test Executions, Test Plans.

Currently, you can't add custom fields to Test Runs since they're not JIRA issues.

You may add CFs to Tests, however they’re not copied to the Test Run. Custom fields at Tests can be seen as complementary, non-relevant information

Xray provided custom fields

As mentione in Custom Fields and Screen Configuration, whenever you install Xray it will also add some custom fields.

Requirement Status

dd

dd

dd

Don’t reinvent the wheel; use standard fields, such as labels and components, and avoid creating custom fields without need
Remember that custom fields added to Tests are not copied to Test Runs, so they’re volatile/informative only; they’re not part of the specification