Steps :

1 .  Create the select option

2. At Initialization, event restrict the select option value by using  FM - SELECT_OPTIONS_RESTRICT

*// Constants
CONSTANTS :
  c_s TYPE char1      VALUE 'S',
  c_i TYPE char1      VALUE 'I'.

*// Data Declaration
DATA :
  wa_gr_restrict TYPE sscr_restrict,
  wa_gr_optlist  TYPE sscr_opt_list,
  wa_gr_ass      TYPE sscr_ass,
  l_classnum     TYPE klasse_d.

SELECT-OPTIONS: s_class FOR l_classnum  NO INTERVALS.

INITIALIZATION.
*// Restricting the VALUE selection to only EQ and 'BT'.
  wa_gr_optlist-name = 'OBJECTKEY1'.
  wa_gr_optlist-options-eq = 'X'.
  APPEND wa_gr_optlist TO wa_gr_restrict-opt_list_tab.

  wa_gr_ass-kind = c_s.
  wa_gr_ass-name = 'S_CLASS'.
  wa_gr_ass-sg_main = c_i.
  wa_gr_ass-sg_addy = space.
  wa_gr_ass-op_main = 'OBJECTKEY1'.
  APPEND wa_gr_ass TO wa_gr_restrict-ass_tab.

  CALL FUNCTION 'SELECT_OPTIONS_RESTRICT'
    EXPORTING
      restriction            = wa_gr_restrict
    EXCEPTIONS
      too_late               = 1
      repeated               = 2
      selopt_without_options = 3
      selopt_without_signs   = 4
      invalid_sign           = 5
      empty_option_list      = 6
      invalid_kind           = 7
      repeated_kind_a        = 8
      OTHERS                 = 9.

Output