How to read the difference between t15 and t15-T15 toners

Wella toners are usually used as a way to apply a tone to products.A t15 will have a slightly darker tint than a t15, but they are interchangeable and often used interchangeably.T15 is a lighter tint that can be applied to the base coat of products.T16 is a darker tint that cannot be applied directly