This line 

> The symbol may naturally be thought of as an element in the K-theory
> of X

appears in the [nLab page on principal symbols][1] for differential operators. What does this mean? Are they talking about K-theory or K-homology? How does one produce a class from the symbol of the operator?


  [1]: https://The%20symbol%20may%20naturally%20be%20thought%20of%20as%20an%20element%20in%20the%20K-theory%20of%20XX