There is a direct correlation between a segment and a datafile.

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A segment in an Oracle database is a set of structures that contains all the data for a specific logical storage structure within a tablespace (e.g., a table, index). A datafile, on the other hand, is a physical file on the disk that stores the data for the database. While segments are logically contained within tablespaces, and tablespaces are physically stored in datafiles, there isn't a one-to-one correlation between a segment and a datafile; a single datafile can contain multiple segments, and a segment can span multiple datafiles if the tablespace it resides in is spread across more than one datafile.

There Is a Direct Correlation Between a Segment and a Datafile