The first position we will cover is the patrol carry (i.e. sling ready) position.
Public domain image, courtesy the Department of Defence. Click on image to enlarge.
In the above image, we see US Army soldiers from the 4th Battalion, 23rd Infantry Regiment, 5th Brigade Combat Team, 2nd Infantry Division and ANA soldiers on patrol in a village in Afghanistan. Note the position of the men using their slings to support the weight of their rifles. This is the most comfortable carrying position when carrying a rifle for long periods of time, such as on patrol duty. Notice how each soldier ensures that his rifle is pointing in a safe direction away from his fellow soldiers.
The next position we will look at is the low ready position.
In this position, the soldier holds his rifle already at the shoulder, but with the barrel pointed safely downwards, with the finger close to, but not on the trigger. This position is used especially when approaching an area with known threats, as it allows the soldier to quickly transition from this to a shooting position. However, since the weight of the rifle is supported by the arms, it isn't suitable for carrying a rifle like this for long periods of time.
The final position we will look at is the high ready position.
In this position, the soldier holds the rifle to his shoulder, but aims the rifle barrel upwards. This is more useful when the soldier is in a close group with other friendly soldiers and it is not possible to point the rifle barrel downwards safely. It is also handy if the soldier needs to reload or use the support hand for other purposes, such as opening doors. Like the low ready position, this position also allows the user to quickly transition to a shooting position.