I see no reason to consume sizeof(int) for a boolean variable) Of course it’s always 1 byte.
“Last changed variable in chain” means last changed replicated variable in function. It’s not a term, it’s just an explanation. For example if you have three replicated variables in your class and you set their values during BeginPlay you need to have replication notifier only for the last changed one. To the monent of call of replication notifier on client, all replicated variables already up to date, since they was changed on server before the last changed variable.