[Gaalsien]

The advanced shield generator is a cert for the suit that speeds up the time it takes for the SUIT's shields to start regenerating after taking damage.

The Nanite Mesh Generator increases the rate at which the Heavy's ability, which is the secondary overshield, regenerates.

I can see how it can be confusing, as the descriptions are inconsistent.

The Nanite Mesh Generator is an upgrade to the standard Heavy shield ability that you start off with. There is also the Resist Shield cert, which swaps out the NMG for a shield that reduces damage taken, and an Adrenaline Shield, which I think just recharges when you get kills.