The Listen links are little CGI scripts which give you the best chance of staying connected. They output a playlist of the stream URLs for each of the S E B relays, in priority order, and communicate the proper MIME type to your browser.
The standard Listen link outputs an M3U playlist – a lowest-common-denominator standard that works with most Audio Players.
These are the playlist variants which S E B supports;
Depending on your Player of choice, some links may be more suitable than others.
The following is Archival Content
For a long time, the standard S E B Listen link was in PLS format. Although it claims to be “a more expressive playlist format than the basic M3U playlist,” it is not as widely supported.
The Extended M3U Playlist format is not a true “standard”, but it is at least called out in RFC-8216 under 4.3.1.1. EXTM3U, which itself cites Nullsoft, Inc. as the originator.
In the long run, PLS has remained more proprietary. I eventually switched the default to M3U, once that became clear. Although, here is a curious Winamp Forums discussion from 2005 which speaks to the fact that
EVERY MP3 player can handle it
Note that this discussion happened long before the term “mansplaining” came into common parlance.