The 'User Is In Custom Field' check will verify that the current user is in the specified custom fields, (typically a multi select/user picker custom field) Note, this check only applies to custom fields.
You must specify the field that is the prerequisite for this transition, and whether the user must be in the custom field or not, when configuring the check. For example:
For information on how to configure a check for a workflow transition in JIRA, see the JIRA documentation.
A workflow is configured so that the 'Close' transition has the 'User Is In Custom Field' check where the user must be in the 'User Select' field. A user viewing an issue in that workflow will not see the 'Close' transition unless their username has been selected in the 'User Select' field.
Supported Field Types
In its different modules (especially those for workflows), the JSU app supports many different field types. System fields, as well as custom fields.
However you should be aware, that not all field types are supported. Also not in all combinations. We think we cover the most important field types and still are continuously adding and improving which and how different field types are supported. But the one you need, might just not (yet) work. Some custom fields of other third party app might never get supported.
For that reason you should always test anything you do with the JSU app with fields. Before you buy a license for JSU, try it with a free evaluation license, if it works for you.