The BBFC are the company that certify films and give them their classification. Different certificates will have different guidelines.
U- No discriminatory language or behaviour unless clearly disapproved of. Scary sequences should be mild, brief and unlikely to cause undue anxiety to young children. The outcome should be reassuring. Mild sexual behaviour (for example, kissing) and references only (for example, to ‘making love’).
PG- Frightening sequences should not be prolonged or intense. Fantasy settings may be a mitigating factor. Sexual activity may be implied, but should be discreet and infrequent. Mild sex references and innuendo only. Moderate violence, without detail, may be allowed, if justified by its context (for example, history, comedy or fantasy).
12 - Moderate physical and psychological threat may be permitted, provided disturbing sequences are not frequent or sustained. Dangerous behaviour (for example, hanging, suicide and self-harming) should not dwell on detail which could be copied, or appear pain or harm free. Easily accessible weapons should not be glamorised. Sexual activity may be briefly and discreetly portrayed. Sex references should not go beyond what is suitable for young teenagers. Frequent crude references are unlikely to be acceptable. Moderate violence is allowed but should not dwell on detail. There should be no emphasis on injuries or blood, but occasional gory moments may be permitted if justified by the context. Sexual violence may only be implied or briefly and discreetly indicated, and must have a strong contextual justification.
15- There may be frequent use of strong language (for example, ‘fuck’). The strongest terms (for example, ‘cunt’) may be acceptable if justified by the context. Aggressive or repeated use of the strongest language is unlikely to be acceptable. Sexual activity may be portrayed without strong detail. There may be strong verbal references to sexual behaviour, but the strongest references are unlikely to be acceptable unless justified by context. Works whose primary purpose is sexual arousal or stimulation are unlikely to be acceptable. Violence may be strong but should not dwell on the infliction of pain or injury. The strongest gory images are unlikely to be acceptable. Strong sadistic or sexualised violence is also unlikely to be acceptable. There may be detailed verbal references to sexual violence but any portrayal of sexual violence must be discreet and have a strong contextual justification.
18- Sex works are works whose primary purpose is sexual arousal or stimulation. Sex works containing only material which may be simulated are generally passed ‘18’. Sex works containing clear images of real sex, strong fetish material, sexually explicit animated images, or other very strong sexual images will be confined to the ‘R18’ category. Material which is unacceptable in a sex work at ‘R18’ is also unacceptable in a sex work at ‘18’.
All these things contribute to the certificate of the movie, and they must keep in these guidelines for the movie to be given, if not, it can be changed and the target audience would need to wholely change aswell.