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Of all of the available primitive types defined in the openEHR Support IM specification, only a few are relevant to archetyping.
openEHR does not define primitive types as such, since it assumes the existence of primitive types from implementation technologies. But since such types are needed ubiquitously in specifications, it provides a set of partial class definitions that it assumes. These definitions support the openEHR specifications. For example, there is a String type, and in the specifications, some kind of 'empty' function is often needed. So the openEHR Support IM assumed_types package defines String and a few functions, including is_empty() on it.
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