Both statements are true.
As the last guy pointed out, with rifle dies the die sizes the neck then the neck is pulled back over an expander ball. The die sizes the neck smaller than what you want, then the expander ball determines the final neck diameter. This is not the same thing as flaring the case mouth.
With pistol dies, the case mouth is flared in a separate die. The sizing die only sizes the case.
In a rifle die the neck is resized and then pulled back over the expander ball (part of the decapping pin) in the sizing die.
With pistol dies, expanding the case mouth is done with a separate die. With Dillon dies this is also the die where the powder dump takes place. The sizing die does not flare the case mouth in either case.
If you look at die sets for sale (not Dillon), you will see typically pistol die sets are three die sets and rifle dies are two dies. This is why. With Dillon, the dies are for a progressive loader, so you still need a third die where the powder drops (basically just a hollow tube that screws into the press like a die. Again, with pistols this die flares the case mouth and the powder flows through it. With rifles it just drops powder.
If you are loading cast bullets in a rifle case the case mouth needs to be flared to seat the bullet but this again is a separate die that typically does not come with the die set. It is a separate specialty die. With jacked bullets in rifles the case mouth is not flared.