__EXP_COUNTER__ has a stable value in a given token replacement sequence (right hand side of a macro).
__COUNTER__ __COUNTER__ __COUNTER__ might give you 42 43 44, whereas __EXP_COUNTER__ __EXP_COUNTER__ __EXP_COUNTER__ will produce 73 73 73.
We can imagine that every macro has a hidden parameter:
#define MAC(A, B, C, __EXP_COUNTER__)
we don't pass this parameter when calling the macro; the macro expander does that, and it passes an integer token whose value is incremented for each such call.
__COUNTER__ __COUNTER__ __COUNTER__ might give you 42 43 44, whereas __EXP_COUNTER__ __EXP_COUNTER__ __EXP_COUNTER__ will produce 73 73 73.
We can imagine that every macro has a hidden parameter:
we don't pass this parameter when calling the macro; the macro expander does that, and it passes an integer token whose value is incremented for each such call.