DDU's are consumed when a metric you desire isn't available. For example, if a metric such as "Service Availability" isn't present, you can create that metric but it will cost you DDUS.
Another Example would be if you want to formulate the number of Host Units a host is using, you can formulate a metric based off the Ram Size, this too will cost DDU's.
Its important to note, DDUs are mainly used when a metric isnt there and you create one manually. If you have a metric present but want to add in operators such as Multiply it by .05 to gain fiscal usage, that wont consume DDUS as the data/metric is already in Dynatrace. An example of this is DPS licensing. If you want to know how much Half Stack is costing your DPS, you can count the number of hosts in Dynatrace that has half stack, then multiply by the fiscal cost... say $100. This formula isn't creating a metric as the metric exists, you are just adding a query segment to add the cost value as the metric. Top which you can then use custom metric event (Custom events for alerting) to then alert teams when the spend goes over, or a metric breaches your defined threshold.
-Chad
