All 7 Uses

principal
in
The Hunt for Red October
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It also cast aside the objective measure of justice and ethics which, he decided, was the principal legacy of religion to civilized life.†
Chpt 3. *

principal = most important; or person that is most important
He selected the Higher Naval School for Underwater Navigation, named for Lenin's Komsomol, VVMUPP, still the principal submarine school of the Soviet Union.†
Chpt 3.
Sir, that's more a psychological question than a technical one, and I deal principally with technical intelligence.†
Chpt 6.

principally = primarily or most importantly
Though principally designed as antiship weapons, the third of them believed to carry nuclear warheads were sufficient to devastate the cities of the East Coast.†
Chpt 7.
The SOSUS receptors were principally laid at shallow-water choke points, on the bottom of undersea ridges and highlands.†
Chpt 10.
Hunting the submarines was principally the job of the P-3C Orion.†
Chpt 10.
The gruffness that was part of his job as Mancuso's principal on-board disciplinarian also shielded more insecurity than he would have cared to admit.†
Chpt 17.

principal = most important; or person that is most important

Definitions:

(1) (principal)
most important; or person who is most important
The exact meaning of principal can depend upon its context. For example:
- "the principal idea" or "the principal performer" -- the most important
- "the principal of the school" -- the person in charge
- "a principal of a company" -- an owner or person with controlling authority
- "the principal in a transaction" (as contrasted to the agent) -- the person on whose behalf an agent acts
(2) (meaning too rare to warrant focus)
See a comprehensive dictionary for specialized senses in finance, architecture, law, chemistry, and music.