This article will give you a solution for the following requirement: I want to create a user task in which a user can select records which will be later used in the workflow process.
Before reading this article you should be aware of the UserService API.
Please see links in "reference" section.

Solution

As the workflow user task service, we use a service extension in order to add an output parameter that will contain the selected records.
The following article explains how to achieve this:
https://support.tibco.com/s/article/Extending-a-built-in-service-to-add-input-output-parameters

Then we have implemented a service without display (UserServiceExtended) that will be launched from the user task that uses the default service extension.
When launched, this service:
- Retrieves selected records
- Puts their primary key values, separated by a ";" character, in a data context output variable
- Completes the interaction (displays the accept button)

This service is declared so it can only be displayed/executed in a workflow context.
In our solution, we chose to implement a toolbar so that it can be launched with a separate button.

Result

Before launching the service:


After launching the service:


Data context variable value:

How to retrieve selected records in a workflow user task

User services documentation:

• Overview
• Quick start
• Implementing a user service
• Declaring a user service

Knowledge base article:

• How to use UserService API

https://docs.tibco.com/pub/ebx/5.9.8/doc/html/en/Java_API/com/orchestranetworks/userservice/declaration/UserServiceExtensionDeclaration.html
https://docs.tibco.com/pub/ebx/5.9.8/doc/html/en/Java_API/com/orchestranetworks/userservice/UserServiceExtended.html
https://docs.tibco.com/pub/ebx/5.9.8/doc/html/en/Java_API/com/orchestranetworks/interactions/SessionInteraction.html#complete-com.orchestranetworks.interactions.InteractionHelper.ParametersMap-