It defaults to the director's stream, if no value is passed.
You cannot pass multiple stream IDs to the &broadcast flag; just one. If needing more, consider using the &novideo flag instead
You can imitate the &broadcast parameter using &novideo=PC1,PC2&noheader&nopreview&style=1
While the &broadcast flag is great for reducing the load on guests in a room, it will put all the load onto the director instead.
Consider using NVEnc or other hardware-encoders to encode any RTMP streams in your studio software to reduce CPU load there
Make sure you have a capable computer; an AMD 5900x CPU is recommend for most users using this mode, allowing for medium-sized group rooms with some headroom to spare.
A quad-core computer might only be able to support 1 or 2 guests adequately in this mode
Consider using the &webp flag to reduce the CPU load on the director further; this will lower the quality that the guests see however.
If you would like the guests to see even higher quality video, consider using &trb=2500 as an option to greatly improve the video quality. This also will greatly also increase the load on the director, so good internet and a powerful CPU will be needed
Using a service like meshcast.io, along with the &website parameter, can also greatly reduce load on the director and guests, but this comes at the cost of added latency usually.
If you'd like to understand how Meshcast.io can help reduce CPU load greatly, leveraging VDO.Ninja's group broadcast-feature, you can see this video: