Found following from their faq section
WHAT IS XAPK?
XAPK is an Android application package format. An XAPK file should consist of at least one APK file, which is the skeleton of the app, and an OBB file, which is the extra data file that keeps the app running. An XAPK's file name always ends in the file extension .apk. With apkpure XAPK Installer, you can directly install all APK and XAPK files with one tap. Also, the latest version of XAPK Installer supports multiple APKs(App Bundle) files installation. With this, it allows you to install the latest format of XAPK files.
- XAPK file = APK file + OBB data file
- XAPK file = Multiple APKs file (App Bundle)
- XAPK file = Multiple APKs file + OBB data file