The "Basic" package will replace up to 4 "primary" instruments, and up to 5 "non-primary" instruments. The Tachometer is always one of those, and with the "Basic" model there is no Manifold Pressure gauge available (the "Premium" version has that).
As I understand it, "Primary" means "required" instruments, such as oil pressure and oil temp (which are two of the available "primaries" you can use. The "non-primary" instruments are either non-required instruments, or ones that do NOT replace the factory gauges.
The "Premium" package will replace up to 7 "primary" instruments, and adds another 5 "non-primary" instruments. The primary instruments always include Tach, EGT, and CHT. You can add up to five additional primary functions, selected from this list: MP, Fuel Flow, Fuel Pressure, TIT, Oil Press, Oil Temp, L&R Fuel Level (counts as 2 functions), Main Fuel Level, Aux Fuel Level, Vac, Volts, or Amps. The non-primary functions can include any of the following: OAT (F or C), HP (requires M.P.), Flight Timer, Engine Time, Tach Time, Local Time, Zulu Time, Volts, Carb Temp, Amps, Fuel Flow, Hydraulic Press, G-Meter, Cabin Altitude, Cabin Differential Pressure, IAT, CDT, and CO Detector.
There's a lot more to this, but you get the general idea. The "Premium" system adds a LOT of capability for only a small increment in price. It's what I went with when I put one in the Commander 114 that I used to own. It was fantastic, and I highly recommend this solution. I was initially a bit worried about the readability of the display, since I wear reading glasses, but after seeing the display at a fly-in, I realized it was crystal-clear and very sharp. I will probably install one in the experimental Bearhawk Patrol I'm building.