The definition of principal

[Mike Flynn]

In the OASIS docs, I see this:
The optional <Subject> element specifies the principal that is the subject of all of the (zero or more)statements in the assertion.
In the glossary it is defined as this:
A system entity whose identity can be authenticated. [X.811]

What exactly is meant by system entity?  Does the principal in an assertion have any association with the user specific data being passed as attributes?
Thanks.
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