Depending on type of the modification instruction, different body properties are allowed:
https://docs.camunda.org/manual/develop/reference/rest/process-instance/post-modification/#request-body

By OpenAPI spec, we can provide the type as a Discriminator Object and describe the request body parameters depending on it:
https://github.com/OAI/OpenAPI-Specification/blob/master/versions/3.0.2.md#discriminator-object