When using DAL with RDB, each user should have a distinct VMS account, including unique UICs and login directories. With a DBMS like Oracle, which has its own user account validation system with the DBMS, you don't need distinct VMS accounts.
However, RDB relies on VMS to let it know who is using which record. If RDB can't distinguish between the different accounts, it can't tell which record is being updated, so it locks all records opened by the current user. Because, in this case, everyone is effectively the same user, all records are locked for everyone until the commit is complete.
This issue was addressed in RDB 4.0 and DAL 1.3.