After 1906 the declaration of intent contains preliminary information and may have incorrect dates of arrival and anglicized names. However petitions after 1906 have information that has been verified and matched to an immigration record. An immigrant who arrived after June 29, 1906, could not naturalize until the government located their immigration record (a passenger list).
Since 1906, after an immigrant filed a Declaration of Intention or a Petition for Naturalization in a naturalization court, the Bureau of Naturalization was called upon to provide a certification of the immigrant's arrival record. The certification, called a Certificate of Arrival was sent to the courthouse.
So when researching, think of opposites - AFTER 1906 you want the Petition. Before 1906 you want the Declaration of Intent
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