In terms of interoperability with other systems openIMIS follows the HL7FHIR R4 standard. See more here information on the mapping openIMIS FHIR R4 Overview Page
Diagnosis coding in the system is uploaded at the time of configuration of the system. So depending on what standards are applied in a country the scheme operator can upload a diagnosis list of their choice eg. in Tanzania a smaller version of ICD 9 was initially applied which later was shifted to ICD 10.