> I ask because I'm not sure about the grammar aspect, and in my native language (Polish) we have a few ambiguities like that, which I sometimes jokingly write down with an arrow, e.g. "A can be ---substitutes for--> B" or "A can be <--substituted for-- B", to make it damn explicit which direction the word works.

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I think it's confusing for 2 reasons: passive voice and prepositions.

1) Passive voice - instead of saying A substitutes for B (active voice) it says A can be substituted for B (passive voice.)

2) Prepositions - "be substituted" can collocate with "for" or "by" with the opposite meaning!

A <-- can be substituted by <-- B (use B where you would expect A)

A --> can be substituted for --> B (use A where you would expect B)