Multicast in Apache Camel

I'm newbie in Apache Camel, so please forgive me for the stupid question.
I am browsing examples of sending messages using multicast and I don't understand it.

I know that (in the network layer) multicast source sends datagram to specified address from range 224.0.0.0 to 239.255.255.255, to subscribers, but multicast source "does not know" how many subscribers are, only one datagram is sent for anyone of subscribers .

I do not understand either the example from the documentation (https://camel.apache.org/components/latest/eips/multicast-eip.html#_multicast_example) or from here (https://www.javarticles.com/2015/05/apache-camel-multicast-examples.html).
Why (if I understood correctly) the subscribers of the message are explicitly specified ("direct: a", "direct: b", "direct: c")?
After all, in one moment there may be 3 of them, in another time 10 of them, and so on. I don't think I need to change the code and define e.g. "direct:10", am I right?

Does the Apache Camel multicast mean something different than the one from the network layer?

Yes its not the same. Multicast EIP is a way of sending a message to N recipients at the same time (where the number of recipient is fixed/known ahead of time).

yes, you correctly understand that there is difference between network layer multicast and apache camel multicast.

The use case for camel multicast is when you want to send same message to the multiple endpoints. So in example from docs:

from("direct:a").multicast().to("direct:x", "direct:y", "direct:z");

The same message from "direct:a" will be send to all three endpoints. And the number of "destination" endpoints is defined for each route and could be different for different routes.

Note that in case of:

from("direct:a").to("direct:x").to("direct:y")

You are chaining processing of the messages. The result from the "direct:a" will be send to "direct:x" and the result from "direct:x" will be send to "direct:y", so "direct:y" could get different message as "direct:x".