It basically only applies to workers who also own the company they work through.

> a worker who provides their services through _their own_ intermediary to a client

Being able to substitute staff. If you can accept a contract and then send your next door neighbour to do the work then that is a strong signal that you are outside IR35.

This is entirely irrelevant unfortunately. You could be able to send substitute and still be in-scope of IR35. It's ultimately down to the client's risk appetite.