This is a single system that contains three systems of one SM smaller(the "payload"). The systems in the payload are all disabled if the microsystem is, and destroyed if the microsystem is.
The systems in the payload must all be installable in the hull section and level where the microsystem is installed.
Like a weapon battery with mixed beams and launchers, a microsystem is a high-energy system if any system in the payload is, but non-high-energy systems in the payload still operate if no Power Point is allocated. For better resolution, you can model a microsystem as providing three Power Points of the payload's SM when provided with one Power Point of its own SM.
A power plant provides 1/3 as many Power Points. For better resolution, you can model a power plant as providing Power Points of the payload's SM, which can be used for high-energy systems in the payload. For example, in a microsystem containing a fusion reactor and two standard reactionless thrusters, the reactor can power the thrusters by itself.
Engines provide 1/3 the thrust(since they're pushing 3× the mass), and tanks hold 1/3 as much as a tank of the microsystem's SM.
Armor and force screens are "thin", providing 1/3 of the DR they'd normally provide, since they must be spread over a larger area. (This cancels the ×3 DR for a partial force screen.) Cloaking devices similarly provide a mere -3 penalty.
A control room in the payload can control the ship, but at -1 Hnd.
Contragravity lifters, defensive ECM, and engine rooms aren't effective at the smaller size. Don't install them in microsystems.
One macrosystem is a cluster of three systems, which together make up a single system of one SM larger(the "payload"). The payload is disabled if any system in the macrosystem is, and destroyed if any system in the macrosystem is.
The systems in the macrosystem must all be installed in the same hull section, and they have the same location restrictions as the payload - except that if the payload was restricted to core, then only one of the systems is restricted to core.
If the payload is a high-energy system, then all three systems in the macrosystem are - requiring three Power Points to energize. If the payload improves with a second Power Point, it requires an extra three Power Points.
Force screens protect a large volume that contains the ship(or hull section, for partial screens); smaller ships can take shelter in this volume. If you want "thick" force screens, install a regular-sized partial screen in each hull section. Cloaking devices and contragravity lifters similarly affect a large volume instead of increasing raw effectiveness.
For armor, cargo holds, defensive ECM, engines, hangar bays, power plants, and tanks, don't use macrosystems; represent such a situation with three regular-sized systems of the same type.
For weapons smaller than spinal batteries, don't use macrosystems; represent such a situation with the three regular-sized systems of the next-larger battery type. (Which will be a single spinal weapon if installing a larger major battery.)
A control room payload controls the ship at +1 Hnd.
For habitat, open spaces, and passenger seating, you generally shouldn't use macrosystems, but can when the regular-sized systems aren't available. For example, an SM: +7 ship could install a macrosystem with an open space from the SM: +8 column, getting a single open area that takes up three systems.