When in "raw mode" the LEGO defined higher level protocol (i.e. Fantom messages) is disabled and the application program can directly 'read' and 'write' bytes over an existing Bluetooth connection. This means that the user has complete control over the bytes being sent over Bluetooth, which allows the NXT to theoretically connect to a wider range of devices. Normally, though, it won't increase speed by a large amount because the speed is generally determined by the chipset, not the driver itself. Effective coding (whether using normal mode or raw mode) can help reduce latency as well.
We have the full listing of NXT Bluetooth commands available for ROBOTC on our wiki, here
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