The outdoor boiler or hot water furnace are usually built into a self contained structure
(usually resembling a metal clad utility shed) and designed to be set up away from your building(s).
All models can be installed indoors if they meet clearances to combustibles rules.

The outdoor boiler or hot water furnace is usually centrally located and used to to heat
multiple buildings - house, garage, shop, barn, hot tub, swimming pool, grain drying, etc.

The boiler is open - otherwise referred to as non pressurized or open to the atmosphere boiler.
This is the safest system with no chance of explosions.

Benefits include:
1) The furnace is situated outdoors close to the fuel source.
2) No ashes, bugs, bark and smoke in the building(s).
3) No risk of fire to your buildings from hot ashes or a chimney fire.
4) If your alternative fuel source is wood there is less work involved acquiring and handling
wood as you cut the wood in lengths of 28" to 60" (instead of 16" for a wood heater)
5) Eliminates indoor burning that can affect indoor air quality and carbon monoxide levels.
6) Usually lower insurance premiums then having an alternative heat appliance located indoors.
7) The outdoor boiler can provide both central heating to the building and domestic hot water,
compared to stoves that provide space heating to the room in which they are installed.
8) Last but not least is the cost savings for heating your building(s).

Outdoor Boilers are usually constructed to:
CSA Standard B366-1-M91, UL Standard - UL 391 (3rd Edition)
and ULC Standard 726 (7th Edition)