This requirement is intended for identification of status of archive block. The format of DDE reference is following:

= D_2000|ARCH_DATA!blname

D_2000 - DDE Server name (required).

ARCH_DATA - requirement for status of archive block.

blname - logical name of archive block (COM_ID).

Possible statuses (value of reference):

• DATA_OK – the required values from archive are available for processing,
• NO_REGISTERED – archive block of the particular name has not been registered,
• LOADING_ARCH_DATA – data loading from archive,
• NO_ARCH_OBJ – non existing archive block or when registering the block, different object has been chosen than historical one
• NO_UNIQUE_NAME – ambiguous name

The example of DDE reference is - here.