The A0...Ax notation are just constants defined in one of the standard header files
...\mega\pins_arduino.h
#define PIN_A0 (54)
#define PIN_A1 (55)
#define PIN_A2 (56)
#define PIN_A3 (57)
#define PIN_A4 (58)
#define PIN_A5 (59)
#define PIN_A6 (60)
#define PIN_A7 (61)
#define PIN_A8 (62)
#define PIN_A9 (63)
#define PIN_A10 (64)
#define PIN_A11 (65)
#define PIN_A12 (66)
#define PIN_A13 (67)
#define PIN_A14 (68)
#define PIN_A15 (69)