On Musical scores that have the tab under it (ie Power Tab Editor) What is the musical Offset for

Like For example, the song Hypnotize, by System Of A Down, the power tab says the first note is C, well it would be if it was in Standard tuning, dit its in Drop D Half Step, so I looked and I saw offset was on One half step, so I wanna know what offset does, and why does it do that, because when I set offset to zero, Every note is off key!
I don't use it, but I'm guessing it offsets the tab to account for you not being in the same tuning
Actually called Mark!

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