DF-Flag on UDP-based sockets?

[Tom]

Hi list

Regarding ARM 9.18.9 
(https://bind9.readthedocs.io/en/v9_18_9/reference.html#namedconf-statement-edns-udp-size):
"The named now sets the DON’T FRAGMENT flag on outgoing UDP packets."

Tested with BIND-9.18.9, I didn't saw any UDP packets, where the 
"DF"-flag was set on the IP header (true for TCP, but never seen for UDP).

Which circumstands or which queries enforces BIND9 to set the "DF"-flag 
on outgoing UDP-based packets?

Any hints for this?

Thanks a lot.
Tom