Various specifications specify files and file formats. This specification defines where these files should be looked for by defining one or more base directories relative to which files should be located.
The XDG Base Directory Specification is based on the following concepts:
            There is a single base directory relative to which user-specific
            data files should be written. This directory is defined by the
            environment variable $XDG_DATA_HOME.
          
            There is a single base directory relative to which user-specific
            configuration files should be written. This directory is defined by the
            environment variable $XDG_CONFIG_HOME.
          
            There is a set of preference ordered base directories relative to
            which data files should be searched.  This set of directories is defined
            by the environment variable $XDG_DATA_DIRS.
          
            There is a set of preference ordered base directories relative to
            which configuration files should be searched.  
            This set of directories is defined
            by the environment variable $XDG_CONFIG_DIRS.
          
            There is a single base directory relative to which user-specific
            non-essential (cached) data should be written. 
            This directory is defined by the
            environment variable $XDG_CACHE_HOME.
          
      $XDG_DATA_HOME defines the base directory relative to
      which user specific data files should be stored. If
      $XDG_DATA_HOME is either not set or empty, a default equal to
      $HOME/.local/share should be used.
    
      $XDG_CONFIG_HOME defines the base directory relative to
      which user specific configuration files should be stored. If
      $XDG_CONFIG_HOME is either not set or empty, a default equal to
      $HOME/.config should be used.
    
      $XDG_DATA_DIRS defines the preference-ordered set of
      base directories to search for data files in addition to the 
      $XDG_DATA_HOME base directory.
      The directories in $XDG_DATA_DIRS should be seperated 
      with a colon ':'.
    
      If $XDG_DATA_DIRS is either not set or empty, a value equal to
      /usr/local/share/:/usr/share/ should be used. 
    
      $XDG_CONFIG_DIRS defines the preference-ordered set of
      base directories to search for configuration files in addition to the 
      $XDG_CONFIG_HOME base directory.
      The directories in $XDG_CONFIG_DIRS should be seperated 
      with a colon ':'.
    
      If $XDG_CONFIG_DIRS is either not set or empty, a value equal to
      /etc/xdg should be used. 
    
      The order of base directories denotes their importance; the first
      directory listed is the most important. When the same information is
      defined in multiple places the information defined relative to the more
      important base directory takes precedent. The base directory defined
      by $XDG_DATA_HOME is considered more important than 
      any of the base directories defined by $XDG_DATA_DIRS.
      The base directory defined
      by $XDG_CONFIG_HOME is considered more important than
      any of the base directories defined by $XDG_CONFIG_DIRS.
    
      $XDG_CACHE_HOME defines the base directory relative to
      which user specific non-essential data files should be stored. If
      $XDG_CACHE_HOME is either not set or empty, a default equal to
      $HOME/.cache should be used.
    
      Other specifications may reference this specification by specifying the
      location of a data file as
      $XDG_DATA_DIRS/subdir/filename. This implies that:
      
            Such file should be installed to $datadir/subdir/filename
            with $datadir defaulting to /usr/share.
          
            A user specific version of the data file may be created in
            $XDG_DATA_HOME/subdir/filename, taking into
            account the default value for $XDG_DATA_HOME if
            $XDG_DATA_HOME is not set.
          
            Lookups of the data file should search for ./subdir/filename relative to
            all base directories specified by $XDG_DATA_HOME and 
            $XDG_DATA_DIRS . If an environment 
            variable is either not set or empty, its default value as defined by this specification 
            should be used instead.
          
      Specifications may reference this specification by specifying the
      location of a configuration file as
      $XDG_CONFIG_DIRS/subdir/filename. This implies that:
      
            Default configuration files should be installed to $sysconfdir/xdg/subdir/filename
            with $sysconfdir defaulting to /etc.
          
            A user specific version of the configuration file may be created in
            $XDG_CONFIG_HOME/subdir/filename, taking into
            account the default value for $XDG_CONFIG_HOME if
            $XDG_CONFIG_HOME is not set.
          
            Lookups of the configuration file should search for ./subdir/filename relative to
            all base directories indicated by $XDG_CONFIG_HOME and 
            $XDG_CONFIG_DIRS . If an environment 
            variable is either not set or empty, its default value as defined by this specification 
            should be used instead.
          
      If, when attempting to write a file, the destination 
      directory is non-existant an attempt should be made to create it 
      with permission 0700. If the destination directory
      exists already the permissions should not be changed.
      The application should be prepared to handle the case where the file 
      could not be written, either because the directory was non-existant 
      and could not be created, or for any other reason. In such case it 
      may chose to present an error message to the user.
    
When attempting to read a file, if for any reason a file in a certain directory is unaccessible, e.g. because the directory is non-existant, the file is non-existant or the user is not authorized to open the file, then the processing of the file in that directory should be skipped. If due to this a required file could not be found at all, the application may chose to present an error message to the user.
      A specification that refers to $XDG_DATA_DIRS or 
      $XDG_CONFIG_DIRS should define what the behaviour 
      must be when a file is located under multiple base directories. 
      It could, for example, define that only the file under the most 
      important base directory should be used or, as another example,
      it could define rules for merging the information from the different
      files.