According to para 4.37 of the 1993 SNA a holding company is a company without significant production of its own with the principal function of controling and directing a group of subsidiaries. This means that a holding company (an establishment) has internal functions within the corporation - a typical case of an ancillary activity whose treatment is according to the highest value-added of the establishment it serves. However double coding is recommended to enable the measurement of the "real" activity of such establishments.
Since there is no such service as a "holding company service" that could be bought on the market, it is not in the CPC.