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DITA_OT_DIR refers to the default directory that is specified for your DITA Open Toolkit distribution in the Window > Preferences > Oxygen XML Editor plugin > DITA preferences page.

For example, if you are using DITA OT 3.2.1 that comes bundled with Oxygen XML Editor plugin, the default directory is: [OXYGEN_INSTALL_DIR]/frameworks/dita/DITA-OT3.x. You can also specify a custom directory.