"What is seasoned rice vinegar and how is it different from plain rice vinegar?"
Seasoned rice vinegar is lightly sweetened (with sugar, corn syrup, or a combination of sweeteners) and diluted to a lower acidity level with the addition of water. This recipe for Sweet-and-Sour Veggie Pickles is a great way to showcase seasoned rice vinegar.
Plain rice vinegar is brewed from rice. This recipe for Sushi-Rice Salad is a good example of using plain rice vinegar to add flavor to a rice dish.
For more information about using vinegar in recipes, see 7 Ways to Cook with Vinegar.