If I subscribe during a seasonal promotion sitewide, for example 25% off during Black Friday, which discount is applied?

Seasonal discounts sitewide are not valid in conjunction with subscription offers. Every subscription sign-up during a sales period automatically receives the highest discount rate, which is equal to the seasonal promotion for one-time purchase.  

In the case of your example, when you sign up for auto-delivery during Black Friday, the first subscription order will automatically enjoy 25% off without any promo code needed. If you try to enter a sitewide Black Friday promo code at the checkout, it will notify you that "The voucher is not valid". Offers for your coming recharges afterwards will comply with our Subscription Policy with no extra discount.