The practice whereby a seller forces a buyer to take several products rather than just one of them, and hence forces him to take a ‘full line’. It can be used most effectively when the seller has a monopoly in some of the products but has competitors in the others. The full-line force can then be used to extend the monopoly power of the firm at the expense of competing firms. An example of a full-line force would be where colour film is sold ‘process paid’: that is, on purchase of the film, the buyer would also pay a charge for developing the negative. Thus the buyer does not have the option of buying the film separately from the processing.
Reference: The Penguin Business Dictionary, 3rd edt.