A text code is the purest kind: the words are in the dots and the phone simply displays them. Use it for serial numbers, locker codes, a short set of instructions, a poem on a wedding favour, or a coupon code at the till.
Up to around 300 characters is comfortable. Beyond that the grid gets dense and needs to be printed larger. The scan check will tell you when you have gone too far.
Questions
Can I format the text?
Line breaks survive; bold and links do not. If you need formatting, link to a page instead.