Forum:Summoning Curse, is it optional or mandatory cost and why?

I have scoured the wikia pages on Summoning Curse, Cost, Mandatory Cost and cannot find a description of what makes some costs mandatory and others forced.

Specifically, from reading Summoning Curse and card's like it, the upkeep cost at the end would not be optional. There is no "you can" or "if", it tells a mandatory thing to do.

However, the Tips section (not a part of explaining Cost, or even cost related area)says that Summoning Curse cost is optional.

How do we as duelists tell from what cards say that they are/not optional/mandatory.

If there is an update to the card errata, then plz update the Summoning Curse page so we can judge better and have more fun playing, else explain this.