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This is a great question but a bit complex to answer in this type of forum. The quick answer: a full table scan is sometimes a direct path read. It depends on your database version (adaptive direct path reads appeared in 11g), table size, parallelism, and other factors. If you're interested in the topic. Frits Hoogland has done quite a bit of work on this topic, and has an excellent presentation on it: http://fritshoogland.files.wordpress.com/2012/06/about-multiblock-reads-v2.pdf
A Direct Read can happen on behalf of a Full Table Scan or an Index Fast Full Scan
Not every Full Table Scan respectively Index Fast Full Scan will trigger a Direct Read, though.
A Direct Read is a prerequisite to get a Smart Scan
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