An assortment is a small collection of oils for a purpose. For example you can have an assortment for cooking. That doesn't mean you have to use them together but they have the same purpose.
A synergy (or recipe) on the contrary is a collection of oils that are used together to achieve a purpose. For example a synergy for diffusion relaxation. In this case you would use all the oils in the synergy together to achieve to objective.
