The terms and conditions are determined by each individual restaurant, including minimum purchase, eligible days/times, and number of diners. These terms are available on the certificates as well as the Restaurant.com website, and it's up to the restaurant to enforce or ignore their own rules.

Some restaurants are willing to overlook the minimum number of diners if you're buying enough food to make it worthwhile (e.g. two entrees for one person). But most are using the service to cheaply acquire new business... and they're very strict about their rules.