By default, the Bluetooth module is set up to act like a TTL serial slave device, so we can simply plug it in to the GPIO RX and TX pins. Once the module is paired with a device, it will transfer the serial communication over the Bluetooth connection. This allows us to send commands and receive data via Bluetooth and to control Raspberry Pi using a smartphone or PC.
This means you can attach a second module to another device (such as an Arduino) that has TTL serial pins and control it using Raspberry Pi (either by pairing it with another TTL Bluetooth module or suitably configuring a USB Bluetooth dongle). If the module is set up as a master device, then you will need to reconfigure it to act as a slave (see the There's more... ...
