A uniqueness requirement might dictate that the same resource accessed in one attempt will also be the result of any other successful attempt. A uniqueness requirement might dictate that a resource have no more than one locator unless all such locators be the same. A uniqueness requirement might dictate that a resource locator identify exactly one object as opposed to several objects. This document makes no general definition of what constitutes one object, several objects, or one object consisting of several objects. Resource access can fail for many reasons.
Providers fundamentally affect accessibility by moving, replacing, or deleting resources over time.
The frequency of such changes depends on the nature of the resource and provider service practices, among other things. Resource naming standards address the problem of invalid locators. Rights clearance restrictions impossible to express in a locator also affect accessibility for certain user populations. Heavy network load can also prevent access. A locator can both be valid and identify a resource that is unavailable.