This was briefly discussed previously on this sitepreviously on this site. You can find discussions of it in:
- Chapters 6 and 17 of Frenkel and Ben-Zvi: Vertex algebras and algebraic curves.
- Section 2.6.5 of Beilinson and Drinfeld's Quantization of Hitchin's Integrable system (available online)
- Section 2.9.9 of Beilinson and Drinfeld's Chiral Algebras.