This article is from the Star Trek Tech FAQ, by Joshua Bell inexorabletash@hotmail.com with numerous contributions by others.
In "The Enterprise Incident" [TOS], the ship's scanners are able to
differentiate a Vulcan from all of the Romulans aboard another ship.
They are very sensitive, but also take a great deal of time. In many
episodes, this sensitivity is not used. However, to scan at that level
of resolution would take perhaps far longer than the crew has.
In any case, the ACB generators are able to scan the target subject,
and either using some best-guessing or asking the Transporter Chief,
decide what should be transported with the subject.
As one r.a.st.tech poster put it, "One to beam up, hold the bunny
slippers." :)
 
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