If a consumer substitutes a high-alcohol drink with a lower-alcohol drink, they will consume less alcohol in that drinking occasion. If that shift happens at the population level, we can see the harmful use of alcohol reduced thanks to this substitution effect.
Policies that differentiate according to beverage type and alcohol content in ways that nudge consumers towards lower strength products support the substitution effect. Differentiation is an evidence-based approach for regulating alcohol that also has positive impacts on economies and communities, in addition to public health outcomes.
This fact sheet gives more information on how differentiation and the substitution effect work.