Do message and data rates apply to unlimited data?
Almost all cellphone plans come with unlimited text messaging included and buckets of data, so your customers are very unlikely to incur any costs. If they have a pay-per-text plan, then whatever the cost to send or receive texts is, is the rate that may apply.
What does it mean when it says message and data rates may apply?
"Msg & data rates may apply" is a notice that depending on your customer's cellphone plan, their carrier may charge them standard message rates and data charges to receive your text messages or reply to them.
Does unlimited texting use data?
Going by cell phone network's definition of “Data” , the Text messaging doesn't use "data" - if you can make and receive calls, you can send text messages, even if you can't connect to the internet.