Industrial-grade primes are sometimes used instead of certified primes in algorithms such as RSA encryption, which require the user to generate large prime numbers. Certifying the primality of large numbers (over 100 digits for instance) is significantly harder than showing they are industrial-grade primes. The latter can be done almost instantly with a failure rate so low that it is highly unlikely to ever fail in practice. In other words, the number is believed to be prime with very high, but not absolute, confidence.
