Managed lists are just strings, they're not registered anywhere. You can add an order to a custom one by adding it to the Orders
TransientListMembershipComponent (tl;dr the latter in your case) and then persisting the entity using
Implementation-wise, the SQL plugin has pipeline blocks in
IPersistEntityPipeline which create back-references for any managed lists.
Regular list memberships remain two-way (list to entity, and entity to list via the component) and typically represent things that can never change (all orders are in the
Orders list as well as the
Transient list memberships are meant for things that can change, like status lists. As such, the SQL plugin block removes the component from the entity after they create the back-references resulting in a one way (list to entity)
You can find some great examples of changing status + assigning transient list memberships by peeking at the blocks in