Contact kills the (host/spores/target) remains on the outer surface of plant e.g. leaves or skin of plant, acting as a shield for protection. Contact based product only provides and act as barrier to protect against infection so they must be applied before infection.
Contact is harmful for environment and pollutes water/Soil/produce/goes into live stock.
Systemic penetrate into the plant either by root system or through leafs the plants. They move across the leaf almost everywhere inside the system of plant into the vascular tissue and goes in plant and in harvest.
Systemic harmful for all who eats the produce also pollute the environment/soil/water.