Why is the option to limit a search to full text only on some database publication pages?

1.  The full text limiter will only appear if the database has a combination of full text and citation/abstract content.  If the database contains all full text or no full text, the limiter will not appear.

ProQuest Historical Newspapers: The New York Times with Index contains all full text, so the limiter is missing:

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ABI/INFORM Trade & Industry has a combination of full text and citation/abstract content, so the limiter is present:

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2.  The full text limiter will only appear if the database is a standalone database.

ABI/INFORM Trade & Industry is a stanalone database, so the limiter is present:

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ProQuest Central cross-searches multiple databases.  Since it is not a standalone database, the limiter is missing:

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