Dynamic prefixes change by e.g. ISPs can be injected into dhcpy6d during runtime.
An already running dhcpy6d daemon can be updated with a new changed prefix. Whatever script detects a new prefix may call the following:
dhcpy6d --message "prefix 2001:db8"
This call only sends the message to the running dhcpyd and stops after finishing. The prefix snippet will be inserted wherever there is a $prefix$ variable used in an address or prefix definition.
The $prefix$ variable will be replaced wherever it appears in an address or prefix definition:
[address_client] category = range range = 1000-1fff pattern = $prefix$::$range$
With the previous example prefix snippet the result will be addresses in the range 2001:db8::1000 to 2001:db8::1fff.
The same applies to prefixes:
[prefix_client] category = range range = 1000-1fff pattern = $prefix$:$range$::
Just keep in mind that the $prefix$ variable is just meant to be a snippet of a whole address or prefix definition and not as a whole prefix.