A software engineer is developing an automated scheduling and billing system for a drone-based roof cleaning service. The engineer has decided that the following two details are the only essential factors needed to compute the bill and schedule the service:

• The bill is based solely on the horizontal footprint area of the roof (the flat projection area) and not the duration of the flight.
• The price is £2.50 per square metre of horizontal footprint.

State two other critical real-world details about the property, operation, or billing requirements that the developer has omitted in this abstraction.