Take a look at this quote from Kevin Liu’s (author of Craft Cocktails at Home) article below - please be sure to check out the whole thing!

Don’t let the simple names of these bitters fool you: they are far from one-note wonders. Rather, these are full-flavored bitters packed with spices and herbs, though they are formulated more for base notes than to add an aromatic punch.

The ‘sweet‘ bitters taste only mildly sweet, but they add a rich, cotton-candy like sensation to a drink (the effect is subtle, not overpowering). ‘Sour‘ most closely resembles an orange bitters, but with less bitterness on the palate and slightly more acidity than you’d typically find in bitters. It makes your mouth water. The ‘Salt‘ bitters taste the most like their namesake, but with the addition of supporting flavors (I suspect cinnamon and rosemary, but it was hard to tell). I could see it being used as an upgraded version of saline solution.

Of the four bitters I tried, the one that really stood out was ‘Umami.’ Best described in English as ‘savory,’ the umami flavor naturally occurs in seaweed, mushrooms, tomatoes, and some cheeses. As I tasted the umami bitters, I found myself picking out different nuances of those ingredients as the flavors played on my tongue. It was earthy and rich, like a slice of well-aged country ham. I found myself wanting to taste it again and again.

Emphasis his, added links ours.

Please be sure to check out the whole write-up via Cool New Bitters Showcase Salty, Sweet, Savory, and Sour | Serious Eats: Drinks.