The formal process for amending the Constitution is always an expression of popular sovereignty. The informal process of amending the Constitution can be an expression of popular sovereignty, but is not necessarily so.
Popular sovereignty, in this case, means the rule of the people. This is one of the major aspects of democracy. When the idea of popular sovereignty is adhered to, the voice of the people is heard. The people are able to make their opinions known on a given issue.
The formal process of amending the Constitution does this. The people do not get to vote directly on the amendment, but the legislatures that they elect do get to vote. This means that the people have a voice.
In the informal process, the people’s voice is not always heard. In some cases it is, as when Congress makes laws that informally change the meaning of the Constitution. At other times, it is not. An example of this is when the Supreme Court makes a ruling in a case and, thereby, changes the Constitution.
Thus, the formal process is always an expression of popular sovereignty while the informal process is only an expression of popular sovereignty at times.