what block layer parameter events_dfl_poll_msecs used for?

[yoma sophian]

hi all:
in block layer, there is one paramter like below:
/sys/module/block/parameters/events_dfl_poll_msecs

when I search the usage in Document, I cannot find anything.
from the src, it seems firing media_chage at a fixed interval.

is my guess correct?
and is there any furhter debug option I can open to make sure block
layer did firing those events to lower device?

appreciate your kind help in  advance,