There are two ways of assigning the capacity units to our DynamoDB service: provisioned capacity units mode on-demand capacity mode. The default way of operating is with provisioned capacity units. This mode of operation requires us to specify the capacity units that we are going to be consuming from DynamoDB. Essentially, we reserve the capacity on the backend and ensure that the backend always works with the assigned set of capacity units. 

The provisioned capacity mode of operation is useful when we have predictable application performance or when we have a certain SLA or budget that we do not wish to exceed. Provisioned capacity mode is also eligible for the free tier, meaning it is the right approach for any proof-of-concept or small-scale application.

If our traffic pattern is unknown or unpredictable, we can still use provisioned mode with auto-scaling, where we set the minimum and maximum provisioned capacity and the target utilization. The capacity units will increase above the target utilization to allow spikes in traffic and decrease if we are well below the target utilization, helping us to save money on the provisioned capacity.