Direction matters
Sequences are written N-terminus first. The reverse sequence is a different compound with different properties, so direction is not a formatting preference.
Sequences appear either as three-letter codes (Gly-His-Lys) or as single letters (GHK). Both describe the same chain.
What a sequence tells you
From the sequence alone you can calculate the molecular weight, predict the net charge at a given pH, and identify residues likely to cause stability problems. That is why a Certificate of Analysis quotes it: the theoretical mass calculated from the sequence is what a measured mass is compared against.
What it does not tell you
It does not tell you the shape the chain adopts, nor whether it has been modified after assembly — acetylation, amidation, glycosylation. Those modifications change the mass, which is one reason a measured mass that disagrees with the theoretical value is informative rather than merely an error.
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