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DITA_OT_DIR refers to the default directory that is specified for your DITA Open Toolkit distribution in the Options > Preferences > DITA preferences page.

For example, if you are using DITA OT 2.5.4 with Oxygen XML Editor, the default directory is: [OXYGEN_INSTALL_DIR]/frameworks/dita/DITA-OT2.x (or if you are using DITA OT 1.8.5, the default directory is: [OXYGEN_INSTALL_DIR]/frameworks/dita/DITA-OT). You can also specify a custom directory.