Costs are classified as direct costs or indirect costs. Management accountants use a variety of methods to assign these costs to cost objects.
Cost assignment is a general term that encompasses both (1) tracing direct costs to a cost object and (2) allocating indirect costs to a cost object. Exhibit 2-2 depicts direct costs and indirect costs and both forms of cost assignment—cost tracing and cost allocation—using the Tesla Model 3 as an example.

The two types of cost are:
Direct Costs
Example: Cost of steel and tires for the Tesla Model 3
Indirect Costs
Example: Depreciation cost of the Tesla Factory in Fremont where Tesla makes the Model 3 and other models of electric cars
The two forms of cost assignment are:
Cost Tracing
It is done based on material requisition document.
Cost Allocation
It has no requisition document involved.
The direct cost type leads to the cost object using cost tracing assignment and the indirect cost type leads to the cost object using cost allocation assignment. The cost object example is Tesla Model 3.
Managers want to assign costs accurately to cost objects because inaccurate product costs will mislead managers about the profitability of different products. This could result, for example, in managers unknowingly promoting less-profitable products instead of more-profitable products.
Managers are much more confident about the accuracy of the direct costs of cost objects, such as the cost of steel and tires of the Model 3, because these costs can easily be traced to the cost object. Indirect costs are a different story. Some indirect costs can be assigned to cost objects reasonably accurately. Others are more difficult.
Consider the depreciation cost of the Tesla Factory. This cost is an indirect cost of the Model 3—there is no separate depreciation cost for the area of the plant where the Model 3 is made. Nonetheless, Tesla allocates to the Model 3 a part of the depreciation cost of the building—for example, on the basis of a percentage estimate of the building’s floor space occupied by the production of the Model 3 relative to the total floor space used to produce all models of cars. This approach measures the building resources used by each car model reasonably and accurately. The more floor space a car model occupies, the greater the depreciation costs assigned to it. Accurately allocating other indirect costs, such as plant administration, to the Model 3, however, is more difficult. For example, should these costs be allocated on the basis of the number of employees working on each car model or the number of cars produced of each model? Measuring the share of plant administration used by each car model is not clear-cut.