PubSub broadcasting limits - per channel, or per topic? Which one are whispers?

The docs say there are these limitations for broadcasting information on PubSub:

1 message per second per channel
5 KB message size

What counts as a channel? I thought it was for a whole broadcasters stream, but then I saw on the extensions reference page, under Listen(), that:

Whispers are a set of optional PubSub channels that extensions can use.

Are the whisper topics channels or topics? Is there a 1 msg/second limitation on each whisper, or on the whole channel itself?