In the Division, I know what almost all UI elements mean, except the number below the ammo/clip count, shown below as "20" (highlighted section 2). Even the site I stole the image from neglected to explain what this value meant. Can anyone fill the gap in my knowledge? May seem like a stupid question, but I can't find what it means anywhere.
This is the number of rounds in the clip of your 'other' weapon.
As it's quicker to change weapons than reload, it's helpful to know how many rounds you've got left.
To test, fire all the rounds in your first weapon, then immediately switch to your other weapon, and you'll see the counter turns red and shows
Here is a picture of my character using a sidearm with both weapon clips empty.