The purpose Immunity testing of a electrical equipment is to evaluate its resistance to external electromagnetic interference (EMI).
Immunity is categorized as
Electrostatic Discharge (ESD)
High spike of voltage wave form generated by external source such as a human finger conducted through either spark in the air or physical contact
Radiated Immunity (RI)
Electromagnetic interference generated by external source such as a hobbyist ham radio picked up by the radio antenna on the electrical equipment
Conducted Immunity (CI)
Noisy voltage waveform generated by external source such as an external power supply picked up by the power cable to the electrical equipment.
Summary & Conclusion
Electronic product level of resistance towards electrostatic discharge (i.e body or machine static discharge) is tested against ESD standards.
Electronic product level of resistance towards radiated interference is tested against RI standards
Electronic product level of resistance towards conducted interference is tested against CI standards
Immunity of an electronic product in the every increasing electromagnetic noisy environment is essential for product reliability.