How do you manage contacts across business units, and what is the difference between contact count and sendable contact count?

In Marketing Cloud E2.0, contacts are managed at the Enterprise level in Contact Builder. All BUs share the same contact pool — a contact is the same individual across all BUs, identified by ContactKey.
Contact count (billable contacts): The total number of unique contacts in the Enterprise's Contact Builder — every contact record that has ever been created, regardless of subscription status or whether they have a sendable attribute. This is the number Salesforce bills on.
Sendable contact count: Contacts who have a valid email address on a sendable data extension AND are not globally unsubscribed. Only sendable contacts can actually receive email sends. The sendable count is always less than or equal to the total contact count.
BU-level contacts: Each child BU has its own AllSubscribers list and can have its own sendable DEs, but the underlying contact records are shared at the Enterprise level. Unsubscribing a contact from one BU does not unsubscribe them from other BUs unless a global unsubscribe is triggered.