What is the difference between a fixed penalty notice and a penalty notice in law ?

[lonewolfa1]

looking for the reason why a fixed penalty notice is legally different to a penalty notice say you was fined £50 for a civil matter eg dropping litter why cant you just issue a penalty notice and it be legal and what is the argument issuing a penalty notice and claiming its a fixed penalty notice because the fine is fixed at £50 but it clearly states its a penalty notice can you supply proof thanks