What is the purpose of parent_plan_handle in sys.dm_exec_cached_plans?

I am attempting to prove whether the plan cache is caching statements individually within multiple stored procedures with identical statements. I thought I would be able to use parent_plan_handle to see this relationship however it is always returning null. I've checked this on both SQL 2019 and 2022. When I consult the literature , the column is strangely absent. The question: What is the purpose of parent_plan_handle in sys.dm_exec_cached_plans?