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CATIA V5 works best when CATPart files are used to define a single part, and CATProduct files are used to assemble several parts together.
Within a product, the same part can be used (instanced) multiple times. Parts are positioned relative to each other with assembly constraints.
However, CATIA V5 is very flexible and can be used different ways. As you have done, you can define each part as a PartBody, and have multiple parts in a single CATPart file. Working in this mode, you can either model each part in it's final position, or use part constraints to position the PartBodies.
I recommend using CATIA as it was intended; CATParts for individual parts, and CATProducts for assemblies.