https://brighterimagelab.com/sp_faq/difference-boil-n-bite-teeth-whitening-mouthpieces-durabrite-custom-fitted-teeth-whitening-trays/

Although both trays are safe, the precision DURABRITE™ Custom Fitted Teeth Whitening Trays are more precise and more comfortable to wear. These are the exact mouthpieces that a dentist provides. They help you save Hi-Intensity™ Teeth Whitening Gel and are better for people who may have sensitivities to teeth whitening gels on their gums. Also, DURABRITE™ Custom Fitted Teeth Whitening Trays are much easier for you to create since there is no boiling water involved in the formation process.