Add valid URL parts

docs/technology/standards/url.md

@@ -17,5 +17,19 @@ In addition, URLs and URIs in TNA services SHOULD:
 1. be based on user need rather than the name of a policy, scheme or service, which might change
 1. include the year when using a short URL for one-off promotion of an annual event such as `/events/summer-camp-2020`
 1. have trailing slashes
+1. have valid pages at every level of the URL (see [Valid URL parts](#valid-url-parts))
 
 See [URL standards for GOV.UK](https://www.gov.uk/guidance/content-design/url-standards-for-gov-uk) for a more comprehensive list.
+
+## Valid URL parts
+
+An example of a confusing URL: `/catalogue/ref/E/190/558/15/`
+
+This URL implies the following paths should also be valid which is probably not the case:
+
+- `/catalogue/ref/E/190/558/`
+- `/catalogue/ref/E/190/`
+- `/catalogue/ref/E/`
+- `/catalogue/ref/`
+
+A better approach here would be: `/catalogue/?ref=E/190/558/15`