This is an easy one.
SP = Standard Play
LP = Long Play
SLP = Super Long play (sometimes Slow Play)
EP = Extended Play
SP was first, when VHS debuted in the 1970s. I still have Japanese-made tapes from 1977, one of which was used to record the Star Wars Holiday Special when it aired in 1978.
LP was added not long after SP, though I don't remember the exact date.
SLP was added in the early/mid 80s. I still have the manual, warranty card, and the receipt from a Panasonic we bought in 1986 at Highlands, to the tune of $370! That manual refers to SLP mode, with 8 hours recording available on T160 tapes.
EP is indeed the same as SLP. I don't remember exactly when, or why, but SLP was renamed in the early 1990s. It was probably due to confusion with the same letters being used to distinguish the mode. SP+LP=SLP? And if I'm not mistaken, some models actually did accomplish SLP/EP by adding a set of heads to the standard SP recording. The SP was 2-head, while LP/SLP/EP was 4-head. But I'm going from memory here, and that was 30 years ago!
I've also read claims that SLP vs. EP was a Matsushita (JVC and Panasonic) thing. And since JVC invented the VHS format, that would mean that SLP was the official name of the mode. However, again, that eventually changed. Everything has been EP for decades now, Panasonic/JVC or otherwise.
Anything you read about "SLP and EP" is either wrong or bad semantics/grammar. EP is SLP. SLP is EP.
PAL doesn't have SLP/EP. PAL has 3-hour SP, and 6-hour LP. And most of that is to due to the 25fps vs 29.97fps running time. Supposedly, some odd EP/SLP PAL machines existed, but I've never seen one. It's usually the ramblings of a person misremembering something from several decades ago.