It is a rate they attempt to achieve through their actions. They can influence the rate, but not outright set it due to market conditions.
Think of it like setting the temp on a thermostat. You are attempting to achieve a temperature through the use of a device to put energy into a system, but there are other factors that contribute to the actual temperature achieved.
Just to be clear, the Bank of Canada can control the overnight rate if it moves outside of the "operating band" by directly intervening in the market with overnight repo and reverse repo operations [1].
I'd say it's more like adjusting the input voltage to the AC directly than simply setting the thermostat. They have to monitor the results and changing conditions and then make adjustments manually, rather than rely on an automatic feedback loop.
