If it is your job to decide how much money goes to three charities, you are in charge of the allocation of your donation. The money each charity gets is their allocation.
You'll often hear this word used when talking about things like government funding. People often debate the allocation of federal funds. Should tax dollars be used for the military, or for domestic needs like education and healthcare? These decisions are made by Congress and the President, who work together to create a budget to allocate funds appropriately.