Before revision:  The payoff of the eligible asset is assumed to be an arbitrary non-zero positive element, thus allowing for a wide range of choices. In the context of function spaces, these general capital requirements cannot be transformed into cash-additive capital requirements by a simple change of numeraire unless the payoff of the eligible asset is bounded away from zero.
After revision:  The payoff of the eligible asset is a risk-free bond, on the grounds that the general case can be reduced to the cash-additive capital requirements by a simple change of numeraire unless the payoff of the eligible asset is bounded away from zero.