Immunoassays look for antibodies, or more specifically the antibodies that attack specific antigens.
Labelled assay adds a chemical that reacts with antibody-antigen pair, and then looks for signs of this reaction. Essentially, the added chemical labels the antibodies for easy detection.
Unlabelled test does not add a chemical, and tries to detect antibody-antigen interaction directly, by trying to detect electro-chemical aspects of it.
It seems that labelled assays are easier and more common, while unlabelled require some highly specialized equipment.
Source: https://www.creative-diagnostics.com/Immunoassay.htm
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