What is an 'Advert' packet in BLE?

An advertising packet is only sent by the slave device and is akin to a simultaneous 'hello' message and invitation for an incoming connection. There are 4 types of advertising packets:

Give data and allow no incoming connection - used to broadcast small blocks of data.

Give data and only allow a peer to request one more packet of data

Give data and invite connection from a specific address (i.e. a bonded/trusted relationship)

Give data and invite connection from anyone - promiscuous usage

Although a slave can only be connected to one master, it is allowed to advertise types (1) and (2) while in a connection.

For more articles and definitions, see the Laird Knowledge Center.