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This was briefly discussed previously on this site. You can find discussions of it in:

1. Chapters 6 and 17 of Frenkel and Ben-Zvi: Vertex algebras and algebraic curves.
2. Section 2.6.5 of Beilinson and Drinfeld's Quantization of Hitchin's Integrable system (available online)
3. Section 2.9.9 of Beilinson and Drinfeld's Chiral Algebras.