What you are seeing is the area effect of being in a formation and it is given using the Unit & Formation skill in the spear - pole-arm tree. The purpose is to give a bonus to everyone in the party that is also in the formation. The more people, the bigger the bonus.
Details can be found here:
http://lifeisfeudal.gamepedia.com/Unit_and_formationI should point out (in case it comes up, just so there is no confusion ) that this has absolutely nothing to do with Premium Subscriptions.