What are the two key differences between a remote client copy (SCC9N) and a local copy?

What are the three types of client copy and their transaction codes?

What is client 000 for, and can application users use it?

What must be done before a client can be accessed by external systems (e.g. BW extraction, S/4HANA↔CRM)?

What is used to purge old client-tool logs and to delete table snapshots, and what is the default retention?

After creating a client with SCC4, does the client physically exist yet? Which table stores its attributes?

Why must you supply both a client number and a user ID when logging on to SAP?