OPC 10000-3 defines rules for NamespaceUris.
When creating a NamespaceUri for a Companion Specification, the template for Companion Specifications already suggests the URI “http://opcfoundation.org/UA/<short name>”. As a general recommendation, also for vendor-specific NamespaceUris, the use of %-escaped characters is strongly discouraged.
How to create a new NamespaceUri for a breaking change is described in 11.3.