Two substitution options are given per exercise (Option 1 isn't necessarily better than Option 2 — just different).
Good reasons to substitute: no access to the main exercise; it causes you pain; you really dislike it (but love a sub); you genuinely can't feel it working after an honest few weeks (and you do feel a sub).
Less suitable reasons: you simply haven't done it before (watch the demo and learn it); someone's using the equipment (do something else and come back); the main exercise is harder than the subs (don't always pick the easiest option).