Mechanical

From The StarCraft 2 Wiki

Mechanical units are all types of units that require a lot of mechanized equipment to function. Examples include SCVs, Goliaths and Dragoons. If the unit is made of Terran construction, an SCV can repair it. A "mechanical" unit has some type of biological pilot and is not robotic in nature, and so can still be targeted by abilities such as the Queen's Broodling. Mechanical units generally cannot be healed (SCVs are an exception).