Well, I installed one on the main water pipeline leading to my house. All the water I use is Siliphos treated.
I basically use it as a whole house polyphosphates water softening.
Here is an image of my installation, it is a bit messy, someone painted the area so paint splatter went all over the place:
But it may not be the case for you so here are some ideas as to where can you install Siliphos water treatment, provided that you have access to the pipeline:
Before your garden watering system.
Before the service room or basement so washing machine can have softened water.
Before your water boiler - that is a great idea, it extends the life of the heating coil and prevents "brown" water.
Before your kitchen faucet. Prevents rust and sediments from clogging the water outlet.
Basically, where ever you want a scale and rust free water.