Tactical Marines are your standard SM Squad which can be used in a variety of ways to counter or devastate your enemy whether it be a Chaos Marine Squad or a lumbering Dreadnought, they are usually able at holding the line early to mid-game but might be retired by the Sternguard Veterans squad or Devastator Marine squad due to their strength diminishing to powerful units such as Airborne Kasrkins or units that counter it in terms of range or raw strength like Dark Reapers or Howling Banshees, respectively. They are produced in the Chapel-Barracks and the Orbital Relay.
In game Edit
Tactical Marines are pretty flexible in combat, they can be equipped with a variety of weapons. They're not very good for melee but can reasonably hold their own. They can be given a sergeant and either an apothecary, a standard bearer or a skull probe to give them an extra edge.