The prefix in xarab.shx historically denotes an "Extended" or "External" font. The root "arab" clearly indicates its purpose: Arabic language support. You may also encounter related files like xarabic.shx , xarabt.shx (often a TrueType equivalent or variant), or shxarab.shx . While names vary slightly, their core function remains the same.
How to install a new or custom font for use in AutoCAD products - Autodesk Xarab.shx Autocad Font
Xarab.shx contains the glyph definitions that allow AutoCAD to perform this reversal on the fly. This is why copying text from AutoCAD to Notepad often results in reversed gibberish. The prefix in xarab