PV system masking occurs when single and/or multiple system defects, anomalies, faults or failures are introduced or evolve. They negatively impact the condition and health of a plant to produce energy and/or reduce the capability of the system to produce energy, and are not readily or transparently observed. The issues do not usually present themselves initially in performance data.

Source: Technical Masking in Solar PV — What Is Really Behind the Curtain? – Renewable Energy World