In compliance with Cambridge's Code of Practice we are required to retain work for the purposes of Standards and Training material. Work that represents a consistent grade at a key boundary (by consistent we mean that the marks have been awarded evenly across the assessment objectives) may be standardised.
Standardising a script is something that the Principal Examiners do prior to the coordination meeting. This sets the standard by which the whole marking process is aligned. It is also carried out with reference to last years' standards. This is why any work that has been standardised cannot be returned to schools until after the following years' coordination meeting has taken place.
Training material is usually selected on a slightly different basis (the award of marks across the assessment objectives may be inconsistent, for example or the style of work or media used may be particularly useful in terms of training). Training material is used all over the world to train teachers and sometimes examiners.