There are three factions in league of war each with their specific advantage and denoted by a faction symbol:
This is the long range faction and all units have a few extra range points than the other variants. Units of this faction have a bluish-gray coloring.
This is the extra armor faction and all units have a few extra armor points than the other variants. Units of this faction have a tan-brown coloring.
This is the cheap faction and these units have base stats along with the lowest supply cost. Units of this faction have a black coloring.
Typically a unit has three variants with one belonging to each faction. In the case of UR & SR units typically there are two SR variants for each UR unit. In those cases the UR can be counted upon to still have the highest attack and defense ratings and there are of course exceptions to the faction rule where the cheap faction is the UR and is in fact more expensive than the SR variants. In most cases though the faction symbols are accurate.
Certain campaign objectives also call for only using a unit in one specific faction so you need to click on a unit in your company and click on the details. On the details page you will see the faction symbol so that is how you can organize a squad with only that factions units. There are a few units that maybe only have one variant or for one reason or another are faction less. Those units will have a blank in the details where the faction symbol should be and they can be used in any of the faction objectives.