Scale-out (Horizontal Scaling) involves expanding capacity by adding more discrete units, such as physical servers or virtual machines, to a cluster. The scenarios "Add more servers" and "Add an additional two-socket system" both represent adding new nodes to distribute the load.

Scale-up (Vertical Scaling) involves increasing the capacity of a single existing unit by adding resources like CPUs or RAM. The scenario "Add more resources... to existing servers" is the definition of vertical scaling. Similarly, adding processors to a single "four-socket system" increases that specific server's density without adding a new node, making it a scale-up operation.

VMware. "What is Cloud Scalability?" VMware Cloud Topics. Accessed November 21, 2025. (Defines vertical scaling as adding power to an existing server and horizontal scaling as adding more servers to spread workload).

Microsoft Azure. "Scaling up vs. scaling out." Azure Cloud Computing Dictionary. Accessed November 21, 2025. (Distinguishes scaling up as adding resources like CPUs/RAM to a single database/VM vs. scaling out as adding more databases/nodes).

IBM. "Cloud scalability: When should you scale-up vs. scale-out?" IBM Topics. Accessed November 21, 2025. (Identifies scale-up as adding resources to an existing system and scale-out as distributed architectures adding infrastructure blocks).

NIST. "The NIST Definition of Cloud Computing." Special Publication 800-145, Section 2. (Provides foundational definitions for rapid elasticity and resource pooling relevant to scaling strategies).