Asynchronous Connection-Less (ACL) is a communications protocol. It is used as a transmission link used for data communication in the Bluetooth system or as a definition with access code(72bit) + packet header(54bit) + payload + CRC(16bit).
Asynchronous Connection-Less Link (ACL) is used when data is more important than avoiding latency. In this type of link, if a payload encapsulated in the frame is corrupted, it is retransmitted.
A secondary returns an ACL frame in the available odd-numbered slot if and only if the previous slot has been addressed to it.
ACL can use one, three or more slots and can achieve a maximum data rate of 721 kbit/s.
Read more about Asynchronous Connection-Less: Packet Types