Use of WM_SENDTIME field in the Buffer table?

I have created the insert notification service for one table ( Table name ABC). For that buffer table got created in db. Two additional fields are added in that buffer table table, one is WM_ROWID another one is WM_SENDTIME.

When I inserted record into the ABC table, the buffer table is also populated with the same record and WM_ROWID is also populated. But WM_SENDTIME field is not populated and the data type for this field is NUMBER(14).

what is the use of WM_SENDTIME field? Why it is not populated for me? Whether I am missing something… I have searched this particular field in JDBC mannual but I couldn’t find anything.