Acknowledgement Packets (ACK) is used to acknowledge received Update, Query, Reply,SIA-Query, and SIA-Relpy messages and are unicasted back to the sender.
EIGRP ACKs are essentially a Hello with no TLVs included. The ACK number in the packet header whose value is set to the Seq number of the reliable packet being acknowledged.
ACK uses Opcode 5 the same as the Hello Packet.
If a router needs to send a unicast reliable packet and needs to acknowledge a previously received reliable packet, the sequence number of the received packet can be sent along with the outbound reliable packet in the Acknowledgement number field. It would not be necessary to send a standalone ACK in this instance.