It shows up frequently on reservations and is just a way for Disney to warn you that it could happen. It is more likely to occur if you're a large party because most restaurants only have a limited number of tables for large groups. They will do their best to seat everyone together, but if there are 10 in your group(for example) it could happen whereas I've never heard it happen to a party of 4.
This message has been on our group of 3 and we've never been split. Some people have gotten that message as a party of one. Ouch!
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