Considering the scenario depicted in the diagram below, as the title says, can a weak entity type take part in an "is-a" relationship?

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When I've modeled databases, I would use a isType description where you have isA. You appear to have two types of Specs; labelSpecs and itemSpecs. Common data belongs in Specs entity, with type specific data in the appropriate subtype record. Normally subtypes are exclusive, so a Spec would be either a labelSpec or an itemSpec.

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