Single zone completion is one of the types of upper completion which allows producing only one zone. Production tubing is a flow path for fluid from a reservoir to flow to the surface so it protects the casing from corrosion and maximizes the efficiency of the flow.
In a single tubing string completion, typically a packer is set on top of a reservoir so the reservoir fluid can flow up into the production tubing. Types of packers are based on several factors as temperature, pressure, reservoir fluid, etc. Additionally, complexity of tubing and packer installation is driven by objectives.