Note: Unity only creates the main expansion file. This is not the only way to split the app package (other options include third-party plug-ins and AssetBundles), but it is the only automatic splitting mechanism officially supported by Unity. Unity automatically splits the output package into APK and OBB. Refer to the Android Developer documentation on expansion files for more information. If your app is larger than 100MB (which is quite likely for a big game), you have to split your output package into the main part (APK) and the expansion file (OBB).
APK expansion files are used as a solution for the 100MB app size limit in the Google Play Store.
