On a lot of signs, you might see the letter 주 in brackets on the sign. 주 is short for 주식회사 (株式會社). This word means corporation. In other words, if you see 주 on the sign, it means that the business is incorporated. I am not sure whether or not these businesses legally have to put 주 on their sign, or perhaps they are simply proud to be incorporated. From the customer’s perspective, it might make the company seem more legitimate. If you look in the selection of signs and advertisement below, you should see the 주 letter in brackets.
Finally, this business spells out the whole word for corporation, 주식회사, rather than using the 주 letter in brackets.