This is the documentation of JSU for Jira Cloud. If you are using JSU on Jira Server/Data Center, you can find the documentation here.
The 'User Is In Custom in Field' check will verify that the current user is in the specified custom fieldsfield, typically a multi select/user picker custom field.
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:
A workflow is configured so that the 'Approve' transition has the 'User Is In Custom in Field' check. If the current user is not the specified reviewer, the description will be modified to show who performed the review instead.
Supported Field Types