According to the search results, an SGL sticker at McDonalds stands for "single sticker" and is used to indicate that a Monopoly sticker should be included in the bag with the customers meal. This is to ensure that customers who order items without Monopoly stickers, such as side salads or fruit bags, still receive the same number of stickers as those who order items with stickers, such as fries. The SGL sticker is used in the UK, and not all McDonalds items come with Monopoly stickers.