B = Breakfast
D = Dinner
F = Food for Purchase
G = Food and Beverage for Purchase
H = Hot Meal
K = Continental Breakfast
L = Lunch
M = Unspecified Meal
N = No Meal
O = Cold Meal
R = Refreshments
S = Snack

UA uses "G" for United SnackBoxes, because they may include a drink box, green tea, or some other beverage. You'll also see the "G" appear on some *A codeshares (e.g. flights operated by Austrian). The less-familiar letters are primarily used in Europe, where the rarely-used "N" designation warns of the kind of Lack of Hospitality only an uncultured American could appreciate.

In premium cabins, you should be offered a meal on any domestic segment over two hours that departs "during breakfast, lunch, or dinner hours." That basically means you only have to worry about evening or overnight travel. It's all explained on United's web site.