The 1.5 volt battery "boosts" the 12 volt battery, resulting
in 13.5 volts across Rload
The terms "buck" and "boost" are most often associated with
DC to DC converters.
In the real world circuits the "bucking" or "boosting" is not
done with batteries. Generally speaking, a voltage is created
in a coil, and the coil is electronically switched so that the
voltage in it either adds to ("boosts") the input voltage or
opposes ("bucks") the input voltage.