When does soil carbon contribute to climate change mitigation?

In climate change mitigation discussions, technical terms are not always used correctly leading to unintended consequences and exaggerated expectations of the role of soil C for climate change mitigation. Carbon stock, carbon sink - are they the same thing? And does fixing C in the soil, for example by building up soil organic C, automatically lead to climate change mitigation? In public discussions about climate protection, many such concepts often get mixed up.