Before encoding your units and contracts, you need to add your clients.
The distinction is simple: ask yourself "Who do I send my fees to?". That person is your client (your contact person).
A client is very often also an owner, but an owner is not always a client.
Example: a couple of owners, Mr and Mrs Dupont. Both are owners, but Mrs Dupont does not want to handle the management. So Mr Dupont is the client (your contact person).