what is the use of SPL (secondary program loader)

I am confused in clearing my concepts regarding these three questions 1. why do we need a secondary program loader ? 2. in which memory it gets loaded and relocated ? 3. what is the difference between system internal memory and RAM ? as far as I understand via reading links is .. **SPL is required when the system internal memory can not hold the uboot completely so we need to initialize memory using a minimal piece of code called SPL**. Does SPL actually relocate or it is only uboot which relocates itself?