I am confused with the different definitions of the Euler system
I understand from the book "Cyclotomic Fields and Zeta Values" that an Euler system is a map $φ: W\smallsetminus S \to Q$ (algebraic numbers), which satisfies certain conditions. However, in the appendix by Karl Rubin in Lang's cyclotomic fields I encounter a different definition. I would be greatful if someone can help in understanding the definition of Euler Systems.