In all cases, the card or effect will say something along the lines of You may cast this spell for [cost] rather than pay its mana cost, or You may cast this card from [zone] for [cost] instead of paying its mana cost. In other words: An alternative cost is something you can pay rather than paying the spells mana cost
SARs are the primary chase cards in physical Japanese sets, with top examples trading at $100-$500+ on SNKRDUNK
The ability attached to this Psyduck, Panic Jump is a curious one
I know this is a Blue card instead of White, but it really does look like a god-like figure unleashing a beam of heavenly light from within the clouds
Now onto the cards you may not have seen or thought about
Illustrated by Atsuko Nishida, Pikachus original character designer, the card shows Pikachu happily drawing, a playful nod to the art contest that created it and a snapshot of Pokmons earliest creative roots