Abap command meaning

Hi guys

I'm working with the following code:

DATA:
    lo_cmrfw TYPE REF TO /bobf/cm_frw,
    lo_message TYPE REF TO /bobf/cl_frw_message,
    lo_applog TYPE REF TO /scmtms/cm_applog_msg,
    lo_tmrequest TYPE REF TO /scmtms/cl_tm_request.

  DATA:
    lt_message TYPE /bobf/t_frw_message_k.

  DATA:
    lv_incomp TYPE boolean VALUE abap_true.

  lo_tmrequest ?= lo_request.
  lo_message ?= lo_tmrequest->mo_message.
  lo_message->/bobf/if_frw_message~get_messages( IMPORTING
                                                   et_message = lt_message ).

  TRY.
      DATA(ls_message) = lt_message[ severity = 'W' ].
      lo_cmrfw ?= ls_message-message.
      DATA(lv_msgid) = lo_cmrfw->if_t100_message~t100key-msgid.
      DATA(lv_msgno) = lo_cmrfw->if_t100_message~t100key-msgno.

      IF '/SCMTMS/INCOMP' NS lv_msgid.
        SET PARAMETER ID 'MESSA' FIELD lv_incomp.          " Export del indicador de imcompatibilidad.
      ENDIF.

    CATCH cx_sy_itab_line_not_found.
  ENDTRY.

What is the meaning of '?=' in the code: lo_tmrequest ?= lo_request.

Do you have a sap help link that talks about this? I looked for it but didn't know how to look for it.