Secondary attack units?

Which units are secondary attack units as i'm close to getting a gold medal for destroying so many amount of secondary attack units, but i don't know what units are secondary attack units?

Marines

He's talking about the unit category called Ground: Secondary Attack, basically a slightly cheaper and weaker attack unit(by default) which can be in the game if the scenario/map have any unit of that category.
However, I don't think it's affected by Relentless Attack in the game.

Secondary attack units are not in the default units, you have to play in maps with this type of unit used.