Since every byte that is exchanged is numbered, the acknowledgment number contains that next sequence number that the sender of the acknowledgement expects to receive. This is therefore the sequence number plus 1 of the last successfully received byte of data. This field is valid only if the ACK flag is on.
下面的圖也可以清晰的看出來：
Connection termination protocol takes four segments to terminate a connection. Since a TCP connection is full-duplex, each direction must be shut down independently.the receipt of a FIN only mean there will no more data flowing in that direction. A TCP can still send data after receiving a FIN.