In the origin on OASIS operating system, THEOS Multi-User Basic is the dialect of BASIC used in the THEOS Operating System.
THEOS 4.2 used THEOS BASIC version 32 Ver 2.0, while THEOS CORONA (Ver 5.0) uses version Multi User Basic 32/64 Ver 2.1.
Some instructions (like READ, MAT READ, READNEXT and so on) were designed to work on ISAM/LISAM Files directly, which are key:record data base organization.
References
THEOS Multi-User Basic Wikipedia(Text) CC BY-SA