It is common practice to define units of measurement for elements of system dynamics models.
AnyLogic enables defining units for all elements of stock and flow diagrams and moreover for all types of variables (stocks, flows, dynamic variables, variables, parameters) and functions (functions, table functions, schedules).
Defining units is optional, commonly this may help you to check whether the formulas are defined correctly in your model and moreover this may help other users to understand the logic of the model. However, if you are sure that the structure of your stock and flow diagram and formulas are defined correctly, you may omit defining units of measurements for model elements.
In the table below we give some examples of units for fundamental elements of stock and flow diagram  stocks and flows. Stocks are usually quantities of something, while flows are measured in the same units per time.
Stock  Possible units of stock  Inflows  Outflows  Possible units of flow 
bank balance

euros  deposits interest 
withdrawals  euros per month 
housing stock  dollars  housing investment  housing depreciation  dollars per year 
CO2 in atmosphere  tons  tons emitted  tons sequestered  tons per day 
population  persons  births immigration 
deaths emigration 
persons per year 
fuel tank  gallons  refueling  fuel consumption  gallons per month 
To set units of measurement for a model element
When you have finished defining units for all elements of your model, you can check their consistency.
To check units in the model