Non-resident taxpayers have an obligation to have a representative with residency in Spain in the following situations:
section 10 5/2004 Non-residents Tax Act

• When they operate through a permanent establishment
• When they do not operate through a permanent establishment but provide services, technical assistance, installation or assembly work derived from engineering contracts and in general activities or financial operations in Spain not through a permanent establishment.
• Whenever the Tax Officers requires it.
• For residents of countries or territories with no effective tax information exchange agreement in force, when they are holders of assets located in, or rights to be settled or exercised in Spain, other than securities traded in official secondary markets.
However, taxpayers may voluntarily appoint a representative with residence in Spain who can serve as a communications liaison with
the Tax Authorities.
The representatives of non-resident taxpayers operating in Spain through a permanent establishment and of flow-through entities
established abroad with a "nexus” in Spain will be primarily liable for their tax liabilities.