Stocks and flows
There are differences between the concepts of stocks and flows. Stock is a variable measured at appoint of time, whereas, flow is a variable measured over a period of time. Wealth, capital etc are variables which can be measured at a point of time. Therefore, they are stock variables. At the same time, income, output, profits etc are concepts that make sense only when a time period is specified. These are called flows because they occur in a period of time. Therefore we need to delineate a time period to get a quantitative measure of these.
Net Investment -> Flow
Capital -> Stock