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Build-plate (only) support refers to adding supports that go from the build plate up to the supported part of the model (in this case, the lower hinges) ONLY. Contrast this with full supports, which would include build-plate supports as well as other necessary "internal" supports (that do not touch the build plate). For this model, any internal supports would very likely result in a fused cube, while build-plate supports could be used for the lower hinges if you wish. Does that help?