Sorry, I should post a reworded version for clarification. Like Knave, the spells are all "level 1" but can scale to higher levels. Unlike Knave, instead of scaling with PC level, they scale with how much stamina you spend to cast them. So if you spend 1 stamina, you're casting a level 1 spell. If you spend 3 stamina, you're casting a level 3 spell. Spells that say something like "+ L" in them mean you add the level of the spell to the effect, which in this case means adding stamina spent.
I thought about writing it as "+ S" for clarity, but I wanted it to match Knave's notation in case you ditch the rest of the game and just use the spell list for some other OSR game. That might've been misguided!