What is $udf_flags in memcached-api.php?

[red-raccoon-dog]

Hi, please tell me one thing.
In memcached-api.php, functions like set, setMulti, etc. contain $udf_flags as an argument
But in the implementation, this argument does not exist
I followed the history of memcached-api.php to confirm this.
It was added in this commit in 2013, but I cannot read from the merge comments why this argument was added