This article is from the Star Trek Tech FAQ, by Joshua Bell inexorabletash@hotmail.com with numerous contributions by others.
Intra-ship transport in the TOS era was not very reliable. ("Day of
the Dove" [TOS]) Likely, when two compatible transport systems were
available, the surface emitters could "interlock", and the pattern
buffers would synchronize. One transport system would handle the
dematerialization, and hand off the ACB to the receiving end for the
rematerialization. This is much safer and likely requires less energy,
and can be used to get around certain environmental difficulties.
("Realm of Fear" [TNG])
 
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