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Terran Unit Roles (vs. Zerg)
From Liquipedia StarCraft 2 Wiki
Overview of the common uses for Terran units in the match-up against Zerg.
Marines or Hellions
Good all-round units that act as a wall of damage or health against Roaches or Zerglings. Before Stimpack or Infernal Pre-Igniter, Marines and Hellions do not trade too effectively but can cause huge damage once either is finished.
Excellent anti-ground unit splash damage, used for holding choke-positions but weak against flanks. It is typical that a strong push timing is-based around a high number of Tanks. Siege Tanks are target-fired to Banelings, Infestors and Ultralisks.
Strong against Mutalisks, and a huge health tank that can withstand huge punishment. Great for guarding Siege Tanks with Bio, and hold the mid-game for Mech.
Used for Tanking Baneling-hits and anti-Ultralisk/Roach tech. But, do not serve as much utility as Marines, so mass-Marine production is much preferred.
Serve for a brief timing as anti-Broodlord/Corruptor tech but are completely useless after all the Broodlords are dead. Do serve as Overlord-harassers in the early game, if Terran opened Hellion/Banshee as well.
A small Medivac count is kept for healing Stimpack level of damage across the army, but are not produced consistently in favor of Siege Tanks. Medivacs are sacrificed, for harassment drops, once their energy is depleted and then can be re-produced.
In the late game, Terran wants to have more of its supply in Siege Tanks and Thors to have more stopping power against Ultralisks and Broodlords to buy time for Marauders or Vikings; Marines and Hellions are used to deny Zerg a strong economy by sacrificing them for Drones and Hatcheries
Either using the health of Thors to tank Broodlings or Raven's Seeker Missile to deal with the clumps of Broodlord/Corruptor late-game, Terran uses Marine-drops or Hellion Back Stabs to deal fatal damage to Drone lines; so Zerg cannot remax too effectively.
Terran needs to take the fourth or third base to pick a fight with Zerg and force them to attack into Siege Tank lines, or use the mobility of Marines or Hellions.