The proposed SOP calls for:
Tail: Main Tail—gray, evenly barred.
Sickle and Coverts—light gray, barred, some white feathers permissible.
The way I read this is that there is a preference for a completely barred tail but a few white feathers are perfectly ok in moderation. ie they allow white feathers but they are not required.
Sort of like the chestnut is allowed in the crest, back shoulders etc but it is not a requirement. Allows for more variation in how the cocks look.
AH thanks for reminding us!