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With CATIA V5, you choose if you want to apply the material with a link to the material library, or with the material properties copied into the file without a link. Edit+Links and File+Desk will verify if the material is linked or not.
The Link To File option is found in the lower-left corner of the materials list.
(I just did a comparison, and the materials properties added about 20Kb to the file size of the CATPart when I chose not to use the link)