Like in ME1, each class can wear up to a certain type of armor: Soldier get heavy, Vangaurds and Infiltrators get medium, and the other three get light. But instead of the armor being full suits, they are instead modular pieces, seperated into 6 parts: Cheast, Left paldron, Right paldron, Arms, Legs, and Head. Than, have there be three levels of armor type, and assign a numerical value to them, like so:
Light armor pieces = 1 point
Medium armor pieces = 2 points
Heavy armor pieces = 3 points
Note: the head piece does not have a value, and doesnt count towards the armor type.
and from here we have the light, medium, and heavy armors:
Light armor = 5-9 points
Medium armor = 10-14 points
Heavy armor = 15
This will allow people to combine the different armor types to make a specific set to their liking. For example:
Light armor set: Light legs, arms, cheast, and both paldrons = 5 points.
Change that with a heavy chest piece, you now have 4 points in light armor pieces, and 3 points in heavy armor, for a total of 7 points. Swap another piece for medium, and you have 3 points light armor, 2 points medium, and 3 points from heavy, for 8 points total. Despite having a medium and heavy piece of armor on, its still considered light armor because its below 9 points.
And heavy armor will offer more damage resistance and shields, while light will offer more mobility and speed, so using a combination can allow you to build a suit of armor to meet the needs for you particular character build.