What are "rye sours"?

Not the drink.

I'm interested in making this bread: King Arthur Flour Caraway Rye. The recipe recommends the addition of vital wheat gluten or King Arthur's rye bread improver. This is how the King Arthur product is described on the web site: "A blend of rye flavors and sours, diastatic malt, vital wheat gluten (for a good, strong rise) and potato flour (to help combat the "dry crumblies")". I'm disinclined to buy the King Arthur product mostly because their shipping fees to Alaska are ridiculous, and also because I've been meaning to try this: Honeyville Dough Conditioner. But am I missing something without "rye flavors and sours"? The recipe suggests possibly repacing some or all of the water in the recipe with pickle juice, a concept I find rather intriguing, and it gives a range for caraway, which is one the flavors in the King Arthur rye bread improver.