There is no technical definition but this would correlate with a pH of 6 to 7. The term sour is used two ways. A low pH soil is called a sour soil. In addition, poorly drained soil with anaerobic decomposition is also called sour. This is a physical change when the calcium molecules help separate the layers of silicon clay.
Flocculation is more obvious on soils with high cation exchange capacity.
Magnesium molecules nest inside the silicon layers and doesn't flocculate the soil. Most gardeners don't because lime, in the presence of organic acids, drives off nitrogen and reduces the nitrogen value of the compost. Lime may increase bacteria action by adjusting the pH but is not necessary for proper composting. Some gardeners like the convenience of spreading only one product instead of spreading both lime and compost but a mixed product reduces your flexibility to varying rates of lime.