This is an example of how to use a Gas Boiler to produce different water temperatures for 5 zones (High / Low Temperatures) plus a zone for the Indirect Hot Water Tank.
You will notice that in this example there are 3 possible temperatures for the heating zones. There are 3 zones for high temperature that can feed radiators or baseboard convectors at whatever the boiler water temperatures is set to. There are 2 zones for a set temperature that could be used for pex radiant heat zones at different water temperatures.
This is only an example. Actual layouts vary from job to job.