Custom ontologies properties are properties like the other ones, like dcterms:title, etc.
For the datatype, it should be one of the datatypes that are managed by Omeka, here by the module Numeric DataType (https://github.com/omeka-s-modules/NumericDataTypes/blob/7e6ace5df3cada61709113a9e5d7c84e69c44f15/config/module.config.php#L28-L31).