what is meant by X_fieldname in lock objects

Hi,

When ever we create a lock object we will be getting two function modules enque and deque.My doubt is the function modules parameters interface contains some X_fields .

what these fields mean. What is the purpose. for example

FUNCTION ENQUEUE_EVVBRKE.

*"----

""Local interface:

*" IMPORTING

*" VALUE(MODE_VBRK) TYPE ENQMODE DEFAULT 'E'

*" VALUE(MANDT) TYPE VBRK-MANDT DEFAULT SY-MANDT

*" VALUE(VBELN) TYPE VBRK-VBELN OPTIONAL

*" VALUE(X_VBELN) DEFAULT SPACE

*" VALUE(_SCOPE) DEFAULT '2'

*" VALUE(_WAIT) DEFAULT SPACE

*" VALUE(_COLLECT) TYPE DDENQCOLL DEFAULT ' '

*" EXCEPTIONS

*" FOREIGN_LOCK

*" SYSTEM_FAILURE

here lock parameter is VBELN but there is one more field X_VBELN. what is the purpose of this field. Please advise.

regards,

Sam.