Marx is obliquely referring to a point alluded to in Capital III here. Under private capitalism, the private capitalist's profit is indistinguishable from the wages they receive for themselves engaging in productive labour. The degree to which their wages were inflated out of what was actually profit, was illustrated by Marx, by referring to the situation in the worker owned co-operatives, where often former owners who had gone bankrupt, were employed by the workers on a fraction of their former salary.
Similarly, when private capital is replaced by the other form of socialised capital, the joint stock company, the productive labour of management and supervision is separated out to a new strata of skilled workers – managers, technicians and administrators – who take on the function of “functioning capitalist” and whose wages increasingly resemble those of other workers. But, the situation is once more confused here, as Marx says, because the money-capitalists, who own the shares and bonds, issued by the company, appoint Boards of Directors, which sit above these “functioning capitalists” and whose role is to look after the interests of the money-capitalists, as opposed to the interests of the business itself.
The Chief Executives, Chairmen and so on that fulfil this role, in addition to their own payment in share options etc. obtain a share of the profits disguised as salaries and stipends that not only are out of all proportion to any labour they perform, but are normally in inverse proportion to it.
“This also establishes absolutely what unproductive labour is. It is labour which is not exchanged with capital, but directly with revenue, that is, with wages or profit (including of course the various categories of those who share as co-partners in the capitalist’s profit, such as interest and rent).” (p 157)
From this its quite clear from a scientific, capitalist perspective whether labour is productive or unproductive has nothing whatsoever to do with what kind of activity this labour is engaged in, whether it is considered to be socially useful or not. It is not this or that type of activity, this or that type of concrete labour, which determines whether it is productive or unproductive, but solely whether it exchanges with capital or with revenue.
“These definitions are therefore not derived from the material characteristics of labour (neither from the nature of its product nor from the particular character of the labour as concrete labour), but from the definite social form, the social relations of production, within which the labour is realised. An actor, for example, or even a clown, according to this definition, is a productive labourer if he works in the service of a capitalist (an entrepreneur) to whom he returns more labour than he receives from him in the form of wages; while a jobbing tailor who comes to the capitalist’s house and patches his trousers for him, producing a mere use-value for him, is an unproductive labourer. The former’s labour is exchanged with capital, the latter’s with revenue. The former’s labour produces a surplus-value; in the latter’s, revenue is consumed.” (p 157)