People and software making use of URIs assigned outside of their own authority (i.e. observers) MUST NOT attempt to infer properties of the referenced resource except as licensed by relevant normative specifications or by URI assignment policies published by the relevant URI assignment authority.

Relevant normative specifications include the URI specification , registered URI scheme specifications (see IANA URI Scheme Registry) and other normative specifications that specify structured use of URI's or URI components.

For example, based solely on inspection of a URI it is unsound to infer:

that a retrival operation on a URI that ends .html will return an HTML representation of the resource a content-type of text/html.

The scheme component of a URI is the primary determinant of the syntax and semantics of the remainder of a URI. It determines the relevant URI scheme specification which should be registered in the IANA URI Scheme Registry. A URI scheme specification may constrain the sort of resource that identifiers assigned using that scheme reference. For example,

states that The mailto URL scheme is used to designate the Internet mailing address of an individual or service.

In the case of URI's where the hier-part component enables some authority to be identified and that authority (which may be a specification) describes the syntax and semantics of identifiers assigned under that authority, it is possible to use such a description to derive information about a resource. For example, on the basis of the W3C Publication Rules for technical reports it is legitimate to conclude that the following about the resource identified by the URI http://www.w3.org/TR/2003/WD-xquery-20030502/:

The referenced resource is a Working Draft of a W3C technical report.

A common mistake is to make inferences based on the last segment in the path component of a URI. For example, it is common to assume that a URI with a path that ends in .html identifies an HTML document, or that a path ending in .jpeg identifes an image encoded acording to the JPEG specification. These inferences are not licensed by the . When a transfer protocol provides a means to convey media-type information, that is the authoritative determinant of media content-type (and inconsistencies are an error that should not be silently ignored (see )

HTML, XHTML and XFORMs all define mechanisms whereby a client can construct the query portion of a URI from information submitted in a web form. The transformation of completed user agent forms into URI is a powerful means to enable the bookmarking of queries that may then be shared with others or repeated. When the query components of a URI are constructed either by the action of completing a form or executing a script that orignated from the same authority as the constructed reference, the structure of the query remains opaque to the end-user. The user-agent that constructed the URI did so in accordance with a specification it received, as a form or a script, from the authority that resolves the resulting reference. That authority retains all the normal freedoms to organise the way it uses its URI space, to create new query parameters, to change scripts and forms etc.

However, if the authority also publishes the syntax and semantics of the query parameters that it uses, 3rd parties may independently construct URI with particular semantic intent. 3rd parties that find such constructed URI useful will create content and applications that depend upon their structure. This will then either limit the freedom for the assignment authorities to evolve there site (modulo 'Cool URI's don't Change' ) without causing breakages elsewhere or it places a maintainence burden on the dependent applications and content.

defines fragment identifiers in the context of retrival operations where the fragment identifier is resolved in a manner specified by the media-type of the resulting representation. Fragment identifiers are also used in contexts where they merely play a role as part of an identifier in systems where no retrieval is intended. In general, in the absense of a media-type it is not possible to infer properties of a resource or its representations from the fragment identifier component of a URI.

The assignment of URIs from a particular URI scheme should respect any constraints on the type of resource identified imposed by the relevant URI scheme specification. The choice of scheme will be influenced by:

The available resource access mechanisms: HTTP, FTP, filesystem.

A URI assignment authority must operate within the constraints imposed by the delegation path(s) which established its authority. Beyond that the authority is free to organise the URI space under its control in any manner it chooses.

An assignment authority may make use of resource properties in the construction of URI that identify the resource. Properties used in the construction of resource identifiers should be static with respect to changes in the state of the resource, it is not very useful if the identify of a resource varies with its current state.

Query components are often used to carry identifying information in the form of keyword/value pairs. Many references that contain query components arise through the use of forms or from the client side execution of scripts. Both forms and scripts generally originate under the control of the assignment authority without the need to publish a specification of the structure and semantics of the associated queries.

However, URIs containing queries are also propagated as bookmarks and in email messages. Assignment authorities should take care to maintain the URI assignments under their authority, . Changing the spelling or value space of keywords may result in the failure of references bookmarked by others, or software that has been programmed based on any published details of the structure and semantics of queries strings.

The URI specification makes no statement about the equivalence of URI that vary simply in the ordering of query keyword/value pairs. URI that differ only in the ordering of query keyword/value pairs are different URI, however in many cases they may identify the same resource.