What does it mean when something says that UNIX device files are static?

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http://unix.stackexchange.com – I've been reading up on udev. In the "overview" section, Wikipedia states that "unlike traditional Unix systems, where the device nodes in the /dev directory have been a static set of files, the Linux udev device manager dynamically provides only the nodes for the devices actually present on a system". What does it mean when it says "static set of files"? Does it mean that there are always /dev files but they don't always point to a real device? (HowTos)