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The uuid can really be any unique string within the table, but to follow conventions, all OpenMRS uuids should be 36 characters in length and contain no whitespace. (Ideally, all OpenMRS uuids would be in the canonical form, but the uuids in the MVP dictionary, which were created manually, don't follow the 8chars-4-4-4-12chars convention, though they are all 36 characters in length.) Certain modules, such as the HTML Form Entry module, operate under the assumption that a uuid is 36 characters long and contains no whitespace, so if you are manually creating uuids for some reason, you should follow this convention.
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