External Constraints are constraints that are defined by an external stakeholder or are an external factor that needs to be taken into account. External Constraints limit your design choices, they are generally fixed, and any change to the technical specifications can affect your project planning.
Some examples include Market Constraints, Physical Constraints, or Policy Constraints that are imposed by a party that is external to your organization. These external parties can include a customer, the government, or an overseeing rule-making organization.