Try it:
TYPES: r_plnt TYPE RANGE OF z_cds_purc-plant.
METHODS retv_bgsml
IMPORTING
value(iv_from) TYPE d
value(iv_to) TYPE d
value(iv_plnt) TYPE r_plnt
EXPORTING
value(et_itm_bgsml) TYPE tt_bgsml
SELECT-OPTIONS s_plnt for z_cds_purc-plant.
lo_itm_sum->retv_bgsml( EXPORTING iv_from = p_ivfrm
iv_to = p_ivto
iv_plnt = s_plnt[]
IMPORTING et_itm_bgsml = DATA(lt_itm_sum)).