By default, the pool will automatically adjust your miner difficulty over time to have 5 to 15 submits per minute. It's a good trade-off between bandwidth and accuracy.

you can also set a custom fixed difficulty using the password parameter. for example, if you want to set the difficulty to 64, you would use:

-o stratum+tcp://stratum.koinium.it:3433 -u username.device -p x

here are the accepted values for custom difficulty:

scrypt, scrypt-n and neoscrypt: from 2 to 65536

x11, x13, x14 and x15: from 0.002 to 0.512

lyra2: from 0.01 to 2048

if the difficulty is set higher than one of the mined coins, it will be automatically reduced to fit the lowest coin difficulty.