A modular mech platform was the catch-all term for mechs designed to accommodate multiple roles.
Unlike the multipurpose mechs common in first-class mech designs, modular mech platforms weren't designed to fulfill multiple roles at the same time.
Instead, they consisted of a very broad and adaptable base which could be outfitted in several different ways.
Need a ranged mech? Then slap some precision arms to the torso, add some targeting systems to the internals, and equip the resulting mech with a rifle.
Need a knight mech? Rip off the gear previously added onto the platform and instead add some heavier arms as well as extra layers of armor. Add a sword and shield and voila, a knight mech!
Such versatility was very useful to outfits and forces that expected to face a variety of threats. The ability to convert a lot of mechs to a different mech type was incredibly valuable to some customers!
