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There are many ways to get a comma-separated list of values. The simple example above just uses a hardcoded list, but the more interesting use cases are using Substitution variables to create the list. For example:
Using results of a JQL query to create an issue for each issue found - see How to create multiple issues - advanced example
A checkbox custom field - creating an issue for each selected item
A select list custom field - creating an issue for each selected item
Fix or affects version field - creating an issue for each version
Components field - creating an issue for each component