What does the at symbol (@) with IPs mean in ffmpeg and vlc?

In vlc, ffmpeg, and mplayer, I see some docs and forum posts that start an IP with an at symbol, e.g., udp://@239.192.1.31:1234. Other times, I see it used to replace localhost, e.g. rtp://@:8005. What does the at symbol mean in these contexts? Searching for this answer has been difficult.

At least in VLC (and I assume ffmpeg is the same) the @ is used to separate multicast groups from the interface section of an address. So the following address would instruct VLC to to join the multicast group 239.255.1.2 on interface 192.168.1.10 to listen for the UDP stream:

udp://[email protected]:5004

See here.

I think it is for source specific multicast address. I tried it changing to my interface address and other time to transmitter address which sent media to multicast address. Second one worked.