I hope this helps:
A0430 per Optum360 Encoder Pro says " Fixed wing (FW) ambulance (airplane) conventional air service is the transportation by a fixed wing aircraft that is certified by the Federal Aviation Administration (FAA) as a fixed wing air ambulance along with the provision of medically necessary services and supplies. Rotary wing (RW) ambulance (helicopter) conventional air service is the transportation by helicopter that is certified by the Federal Aviation Administration (FAA) as a rotary wing ambulance, including the provision of medically necessary supplies and services."
For A0999 it states, " This code is reported for any ambulance service that is not described by other HCPCS Level II codes. It should be reported as a last resort and only when there is not a more appropriate procedure code. When reporting an unlisted code, provide a detailed explanation of the services in remarks, billing notes, or as an attachment."