In this case, the number of units is calculated automatically and is based on the number of units built into the system.
Why do I see, for example, 2.40 units (quantity) on the invoice?
This can happen based on the rule mentioned above. Let's imagine you have 12 units on June 25th. The calculation is done: 12 / 30 (days of June) * 6 (period 25-30/6/2020) = 2.4 units for June