Defined by the UK sentencing council as follows, and for the purposes of this Code should include all three categories A-C. Category C would include images that show erotic poses without sexual activity, but not include images that show nudity in a legitimate setting. Category A is images involving penetrative sexual activity; images involving sexual activity with an animal or sadism. Category B is images involving non-penetrative sexual activity. Category C is other indecent images not falling within categories A or B.

Per gov.uk