When you click on the list, it will have a sub list attached only to the top level. As in the below list, if "Customer Service" is selected than only the items below "Customer Service" are avalible. So every top level will on show the sub level assisgned to it.
To see if we fully understand, do you mind providing more information about your end goal here? Would this be a suggestion of a feature in HubSpot or something that is currently happening in your account? If it is a suggestion, I recommend you to post in our ideas forum.
The more details and screenshots you can provide, the better the community can help you.