There are many more experienced people on this topic, but since I just learned from them, I will take a stab at it.
It is purely a mineral/vitamin supplement in pellet form. It would be similar to our One a Day vitamins. We eat meals every day, but take a vitamin supplement to even out our daily requirements.
Forage (pellet, cube, long stem), which is a horse's primary meal, contains nutrients. But, depending on the type, cutting, age, environment may lack certain nutrients. The RB compliments the forage.
The same principal applies. You wouldn't swallow a one-a-day in the morning and not eat anything all day. Right? Because the vitamin does not include calories (or substance).
NOW, I'll introduce the word "feed." This may be in pellet or grain form. Horse Feed is not a ration balancer. Horse feed is meant to provide the horse with calories in addition OR instead of forage. Most horse feeds include vitamins and minerals.
The difference between a horse feed and RB is 1) calories and 2) Ingredients. While both have essential minerals and vitamins, horse feed has ingredients that you may not want to give your horse. Such as sugars (which will cause them to be "hot").
You would not normally provide your horse with feed and RB. Feed is like eating a snickers for dinner. Satisfying but not very nutritious. And more calories than you may have wanted.
Forage and an RB is best if you feel you need to supplement.
I provide my horse forage (short grass/pasture, long stem flakes, and soaked pellets) and an RB - Purina Enrich Plus. I have not used anything else, so I can't compare the EPlus, but within two weeks I can see better coat conditioning. I chose EPlus because a local feed store carries it standard.
Lots of people swear by triple crown 30%.
The RB bags do not say "Ration Balancer" from what I can tell. But, they don't call themselves "Feed" either.
If you google Horse Ration Balancer, you will find the common brands. You can then check if they offer it local to you.