E.g. when I plan my holiday I want to create a (master) list "prepare holiday" with sublists (documents, clothes, medicine, etc....) and task items in those sublists.
When inside a particular to-do list, swipe a to-do left or right to define hierarchy.
Also, please take a look at the app's section Settings > Tips and Tricks. It describes various tweaks you can do with the app, including how to manage hierarchy.
Pocket Lists
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