First, there are a few different National and World meets of the APF/AAPF/WPC/AWPC. As far as National meets, there is the APF Raw Nationals, APF Equipped Nationals, and AAPF Nationals. To qualify for the Equipped Multiply Open class at APF Equipped Nationals or AAPF Nationals, you must hit a qualifying total at an APF/AAPF meet before entering the National meets (these are available on the WPC website). There are no qualifying totals for Raw, Bench Only, Deadlift Only, or age divisions at any national meet. In these cases you need have only competed at a sanctioned APF/AAPF meet within one year of the national meet.
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On the World level, there are only two meets, WPC Worlds and the AWPC Worlds. To qualify for WPC Worlds, you must place first through third at one of the APF national meets. Otherwise, you must contact the APF Office to see if there is room on the US team for you as an alternate, after the National meets have taken place. Usually there are alternate spots, especially in the age divisions. To qualify for AWPC Worlds, you must place first through third at AAPF Nationals. Again, you can also apply for an alternate spot on the US team, after AAPF Nationals.