A long series of scientific publications have presented to revisit several aspects of the Radio Frequency Interference (RFI) impact on the airborne L5/E5a band GNSS receiver. The main objective was to define, for the first time, L5/E5a GNSS RFI masks. This long series of publications are culminated with this work where the concept of RFI GNSS mask is reminded and the methodology used to derive these masks is provided. This methodology will be used to derive L5/E5a GNSS RFI masks or interference thresholds to be published in different aviation standards such as ICAO SARPS and RTCA/EUROCAE MOPS (RTCA DO-292 update). The presented methodology will be mainly focused on the L5/E5a GNSS RFI mask and will make reference to other scientific words which are used to characterize necessary inputs to derive the mask, such as the contribution of the different aeronautical interfering sources. In this work, the physical link between the RFI mask interpretation and mathematical model of a RFI impact on a GNSS receiver will be made and the maximum impact allowed to a non-aeronautical RFI source, in terms of power, bandwidth and central frequency, will be explained. The concept of link margin will be introduced as well as the elements determining its value. From the concept of link margin and RFI impact, the L5/E5a GNSS RFI mask derivation methodology will be explained as well as the necessity to inspect the link margin for any signal / basic signal processing function pair.