B-Flex Subli+ Mask is needed to apply B-Flex Subli+ HTV.
Mask is automatically sent with rolls of Siser and B-Flex HTV; it's all rolled up together. However, with printable HTV like Subli+, you need to print your design first and add the mask second.
Subli+ Mask is reusable. Don't just throw it out after one use. If you have some already, use it again!