A club good
(A club good is the one which is non rivalrous but excludable.
Library is best considered a club good. This is because Libraried
are not rivalrous as use of library by one member does not reduce
the amount available for another. But exclusion is possible through
subscription fees.)
considers all private and external costs
(When the total cost is considered from society's angle then in
additin to private costs, external costs are also considered.
Because for society's welfare maximisation, external costs also
need to be considered.)