This is the documentation of JSU for Jira Server/Data Center. If you are using JSU on Jira Cloud, you can find the documentation here.
The 'User Is In Custom Field' condition verifies that the current user is in the specified custom fields, (typically a multi-select/user picker custom field). This check only applies to custom fields. You must specify the required field for this transition and whether the user must be in the custom field. A workflow is configured so that the 'Close' transition has the 'User Is In Custom Field' condition set so that 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.Description
Configuration
Example
See also
You can configure JSU’s preconditions in the following ways:
Negate the precondition result
Combine the result with the result of previous preconditions using the following options:
Discard the result of the previous precondition: Results of the previous preconditions are not taken into account
AND: All of the preconditions have to be true to execute the post function
OR: Only one of the preconditions has to be true to execute the post function
Learn more in Workflow preconditions.