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Formal systems usually included rules for deriving some of the arrangements in the system from others. These rules must depend only on the abstract relations in arrangements, and not on the material form in which they are presented, nor on any meaning that we might associate with them.
In principle, derivations themselves are another sort of arrangement. But we usually think of arrangements as things whose structure can be recognized immediately, while we allow for a more elaborate multistep procedure to check the correctness of a derivation. But this distinction is probably not totally objective.
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