Release 10.1A: OpenEdge Getting Started:
Database Essentials

Extents

Extents are disk files that store physical blocks of database objects. Extents make it possible for an OpenEdge database to extend across more than one file system or physical volume.

There are two types of extents: fixed-length and variable-length. With fixed-length extents you control how much disk space each extent uses by defining the size of the extent in the .st file. Variable-length extents do not have a predefined length and can continue to grow until they use all available space on a disk or reach the file system’s limit on file size.