This radical was developed from a circle with a central dot as the image of the sun. As you can hardly draw circles or dots with a brush, it became the current rectangular shape.
Radical 103 疋 stands for clothing or bales of cloth. In the Hanzi-Trainer, it is used synonymously with the correct 正, because it seems helpful for the explanations and the interpretation clothing/cloth bale would be in conflict with other radicals.
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