What is the Code for "deck footers" that are in a sugar-sandy area?

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If you're in the US, then IRC R507.3 is a pretty safe bet for deck footings.

"Sugar-sandy" sounds like a description of SP soil under the USCS (poorly graded sand, where well graded sand has a wide distribution of particle sizes and better bearing strength), but that's assuming that you're talking about the native soil below the top 12" or so of upper soil. The table under IRC R401.4.1 converts the SP soil classification to a 2000 psf bearing capacity, where you'll need that bearing capacity value for sizing deck footings.

And "tributary area" is a concept that goes along with uniform distributed loading, e.g. "40 psf," to find loads on structural members. For the typical grid pattern of posts, the tributary areas are a bunch of rectangles. For a heuristic, imagine the deflected shape of the flat deck surface under a uniformly distributed 40 psf and now flip it upside down. Under a slow, uniform rain, each post is the drain position for its tributary area. So a post gets half of any beam going to another post. It gets half the joists plus deck spanning from the half beam to any adjacent beam (or ledger). The post gets the full load of any joists cantilevering from one of its tributary beam halves. Etc. Etc.

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