I'm trying to understand how tcp works

I'm interested in detection of packet lose and their retransmissions

while looking tcp_sock structure I've found some data which can't
understand

struct sk_buff* lost_skb_hint;
struct sk_buff *scoreboard_skb_hint;
struct sk_buff *retransmit_skb_hint;
struct sk_buff *forward_skb_hint;
struct sk_buff *fastpath_skb_hint;

I guess that they are pointers in packet queue on sender host

but what are they physically represent?