All documents must be locked in order to be modified. Documents may also be locked to prevent code in other execution contexts from modifying them temporarily. Documents do not have to be locked to perform query (read) operations, and they are never prevented from performing query operations on other documents. Basic document locking is handled automatically for AutoCAD commands, ObjectARX commands, and AutoLISP functions. Modeless dialog and toolbar code, and any commands that need to work outside the active document must manually perform document locking.
For more detailed information on document locking, see Explicit Document Locking.