ESL One Katowice Team Stickers
Along with the stickers is the Pick'em Challenge, to compete in the challenge you use the stickers you purchased prior to the event. The stickers are then used to make your picks for the day. For that day they will be locked, so that means no trading or using stickers you used to make a pick. However at the end of the day and going into the next day they will be unlocked once more and you'll be able to use them to pick the same team.
You can find more info on rules, details, and the trophies available on the official announcement here from Counter-Strike.net.
Will you be buying Stickers for ESL One Katowice? Let us know on Twitter @ESNCS