What does this sentence mean?
org/wiki/Advance_healthcare_directive [An advance directive] is a legal document in which a person specifies what actions should be taken for their health if they are no longer able to make decisions for themselves because of illness or incapacity.
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See https://en.wikipedia.org/wiki/Advance_healthcare_directive
[An advance directive] is a legal document in which a person specifies what actions should be taken for their health if they are no longer able to make decisions for themselves because of illness or incapacity.
It is not a sentence. It is a dependent clause.
An "advance directive" is a legal document. It authorizes another person to do certain things if you are alive, but cannot communicate your requests.
It's not a complete sentence. It's just a subordinate clause.
I don't know what 'an advance directive' means.