The stringList variable is rather specific for fields in Gateway Setup Editor (GSE) that can accept multiple values. You may encounter unexpected results when trying to use it apart from those mentioned in this article.
The most common usage is to use it in the "Expect Row" option under a Sampler's Advanced tab. This could be either at the Expect Rows option directly or under the Dataviews option.
The other possible usage is in the Visibility tab for Hide columns or Hide rows.
In Gateway version GA4.2.0 (GWX-1435) and above, users can also use stringList variables in the inList() function.