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Thank you for your question to History Hub! The general rule-of-thumb is that if the records you seek predate 1914, they will be at Archives I downtown and post-1914 records will be at Archives II in College Park, Maryland. There are, however, many exceptions to that general dividing line, so it would depend on your subject of interest and the type of records for which you are looking. Most non-textual records (audio, video, cartographic, photographic, etc.), for example, are in the possession of the special media branches and can be viewed at the research rooms of Archives II.
You probably want to begin planning your research trip by contacting reference staff who can locate more specific records corresponding to your interests at the corresponding building for those records. For downtown D.C., the e-mail address is email@example.com. For the College Park location, the e-mail is firstname.lastname@example.org.
Materials cannot be reserved in advance, but pull slips can be prepared on your behalf by reference staff to be submitted on the day of your arrival in the research room. For more information on rules and regulations at each facility, you can look on the web at The National Archives in Washington, DC and The National Archives at College Park, Maryland.
Hope this helps and happy hunting!