1988 Subaru GL SPFI, Manual Transmission, Selective 4 wheel drive, has the lo-hi lever.  There is a transistor under the Ignition Coil. The number on it is HF 8311. It looks like it is wired into the ignition coil. Do not understand it's purpose and could it cause a vehicle to stop running? My Subaru repair manuals don't go into this. thanks.

That transistor is used to drive the ignition coil. Mitsubishi used to call this part a power transistor and another manufacturer calls it an amplifier. I don't know what Subaru's proper name for it is. If it goes bad the engine will not start and there won't be any spark. It's not a bad idea to have an extra one in the spare tire well with some other goodies

I tend to call them igniters.  Crazyeights is right.  Failed one = no spark.

It's easy to confirm if you have spark or not.