A core is generally made of a mix of fluff and SAP (hereunder we wil not cover special cores made of other materials or so called "fluffless core"). Usually this mix is “homogeneously blended” (HB): it means that SAP and fluff are in the same ratio everywhere along the core. The reason to have an HB core is because SAP works better when it is well mixed with fluff and this improves core general  performances. Of course other type of constructions are possible altough, with current  cores available on the market today, they do not reach performances that HB cores can.