I like to think of nutrition in terms of three key things: forage, nutrients and calories.
The first and most important thing is forage; that is, hay and/or pasture. A horse should be eating at least 1.5-2% of its ideal body weight in forage daily. Horses are deigned to be eating small quantities of food nearly constantly; their stomachs don't stop producing acid when it's empty like ours do which is one reason why ulcers are so common in horses. Giving a horse 24/7 access to hay or grass is best, but is not always feasible, especially for boarders. When it's not possible, you should try to feed as often as possible (or utilize slow feeders) to avoid long stretches of time without food.
The next key thing is nutrients. Almost all grass is deficient in some mineral or other, whether its a regional deficiency (like selenium in a lot of areas) or specific to the particular pasture due to its agricultural history. On top of that, when grass is processed into hay, it loses some things very quickly (vitamins A & E and omega-3's top that list). A vitamin/mineral supplement or ration balancer
is designed to fill those nutritional holes. Ideally, you would get your hay and/or pasture tested to see what its nutritional profile looks like and match it to a supplement that complements it. But this is not always possible. In those situations you have to do your best to find out if there's a regional deficiency or overabundance in your area of anything in particular and use that as a guide.
The third key is calories. A lot of horses do just fine on hay and a ration balancer (and free choice salt and water!) but some horses need a little something extra. There are a lot of ways to provide extra calories. Traditionally horses were fed grains (like corn, oats and barley) but we now know that horses digest the sugars and starches in grains very poorly and that there are much better alternatives. Alfalfa, rice bran, flaxseed, and beet pulp are all great choices to add to a horse's feed if extra calories are needed; which one to pick depends on the horse's needs and availability in your area.
A lot of the pelleted feeds (such as Safe Choice or Strategy) combine nutrients & calories together. While this might seem easier, you often have to compromise one or the other in this situation; for example, an easy keeper might get too fat if fed enough Strategy to meet his nutritional needs. Or if your horse is doing well on an amount that does provide complete nutrition, but then the amount is decreased when he has to be put on stall rest, you're not only cutting down on calories, but also on nutrients that he still needs.