Defined in macros/scripts/events.asm and engine/scripting.asm:ScriptCommandTable .
Until this document is filled out, the G/S Scripting Compendium has descriptions for most of these commands. It was written for G/S binary hacking and not Crystal assembly hacking, so it's not 100% accurate for pokecrystal.
$06: ifequal byte , script
$07: ifnotequal byte , script
$0A: ifgreater byte , script
$0F: special special_pointer
$12: setmapscene map , scene_id
$19: copybytetovar address
$1A: copyvartobyte address
$1B: loadvar address , value
$1D: writevarcode variable
$1E: writecode variable , value
$1F: giveitem item_id [, quantity =1]
$20: takeitem item_id [, quantity =1]
$22: givemoney account , value
$23: takemoney account , value
$24: checkmoney account , value
$28: addcellnum contact_id
$29: delcellnum contact_id
$2A: checkcellnum contact_id
$2D: givepoke mon_id , level [, item =0[, trainer =0, ot_name , nickname ]]
$2E: giveegg mon_id , level
$2F: givepokeitem pointer
$30: checkpokeitem pointer
$31: checkevent event_flag
$32: clearevent event_flag
$34: checkflag engine_flag
$35: clearflag engine_flag
$3A: warpmod warp_id , map
$3D: readmoney account , memory
$40: pokenamemem mon_id , memory
$41: itemtotext item_id , memory
$42: mapnametotext memory
$43: trainertotext trainer_id , trainer_group , memory
$44: stringtotext text_pointer , memory
$48: refreshscreen [dummy =0]
$4B: farwritetext text_pointer
$4C: writetext text_pointer
$4D: repeattext byte1 , byte2
$4F: loadmenu menu_header
$51: jumptextfaceplayer text_pointer
$52: farjumptext text_pointer
$53: jumptext text_pointer
$58: _2dmenu
$59: verticalmenu
$5D: loadwildmon mon_id , level
$5E: loadtrainer trainer_group , trainer_id
$60: reloadmapafterbattle
$62: trainertext which_text
$63: trainerflagaction action
$64: winlosstext win_text_pointer , loss_text_pointer
$68: setlasttalked object_id
$69: applymovement object_id , data_pointer
$6A: applymovement2 data_pointer
$6C: faceobject object1 , object2
$6D: variablesprite variable_sprite_id , sprite_id
$70: follow object2 , object1
$72: moveobject object_id , x , y
$73: writeobjectxy object_id
$75: showemote emote_id , object_id , length
$76: turnobject object_id , facing
$77: follownotexact object2 , object1
$79: changemap bank , blockdata_pointer
$7A: changeblock x , y , block
$7D: writecmdqueue queue_pointer
$81: musicfadeout music_id , length
$89: passtoengine data_pointer
$8A: newloadmap which_method
$8C: deactivatefacing length
$8F: ptpriorityjump script
$92: reloadandreturn which_method
$94: pokemart dialog_id , mart_id
$97: askforphonenumber contact_id
$9A: describedecoration byte
$9C: specialphonecall call_id
$9E: verbosegiveitem item_id [, quantity =1]
$9F: verbosegiveitem2 item_id , variable
$A3: warpfacing facing , map , x , y
$A4: battletowertext memory
$A5: landmarktotext landmark_id , memory
$A6: trainerclassname trainer_group , memory
$A7: name type , id , memory