Actually, the Pashto term 'اربکیان' comes from kind of voluntary workers who work as per the tribal decisions especially toward the areas in Paktia. And in the recent interpretation, it means the local police who are different from the National Police of Afghanistan. And 'اردو' is the National Army of Afghanistan.

اربکیان, as I explained in the answer to your other question, is a local militia force used for paramilitary activities and as local police.

The word اردو originally comes from Turkish and means 'army'. The Mughols in India were of Turkish origin, their army was called urdu and the mixed language that arose from that army was later called Urdu. In Afghanistan, the army is still called urdu and has nothing to do with the Urdu language.

اردو means national army of a country