The objective of having SDF in LTE network is to provide service level data usage and its charging. LTE network supports two types of bearers known as Non-GBR and GBR bearers.
Non-GBR bearer's parameters : QCI, ARP, MBR, APN-ABMR, UE-AMBR
GBR bearer's parameters : QCI, ARP, GBR, MBR
Similarly, there are mainly two types of SDFs i.e. SDF for non-GBR and SDF for GBR. I mean to say this is highest level of mapping or association for a SDF to an EPS bearer. A SDF QoS doesn't have parameters like APN-AMBR and UE-AMBR while a non-GBR EPS bearer has such type of parameters. Each EPS bearer has TFT "Traffic Flow Template" which may have a list of IP packet filters. For the downlink data, PGW is responsible for mapping of incoming IP packets from external network (SGi) interface to EPS bearers by using SDFs and TFTs.