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The key distinction between a 2-way valve and a 3-way valve primarily lies in their design and functionality pertaining to fluid direction and control. A 2-way valve has two ports and is designed to control the flow of fluid in one of two directions—either allowing fluid to pass through or blocking it entirely. In contrast, a 3-way valve has three ports, enabling it to direct fluid to different paths, combining or selectively blending flows.

Focusing on the correct answer, the exhaust function is a significant aspect of how 3-way valves operate. These valves can manage multiple flow paths, often allowing for exhaust, which is integral in applications where redirecting fluid or managing pressure within a system is necessary. The presence of an exhaust function in a 3-way valve allows for greater versatility and control in applications requiring these specific functions.

The other aspects mentioned, such as flow direction, pressure control, and size and capacity, are indeed relevant in the broader context of valve functionality but do not specifically define how a 2-way valve differs from a 3-way valve regarding exhaust management capability. Thus, the emphasis on exhaust function is what fundamentally highlights the operational differences between these valve types.