disguised packets, blocklist behavior

[Sting0r]

Hey guys, does anyone can explain why the packets have to be changed when casting spells while disguised?
For example in clif_skill_poseffect
 

void clif_skill_poseffect( block_list& bl, uint16 skill_id, uint16 skill_lv, uint16 x, uint16 y, t_tick tick ){
	PACKET_ZC_NOTIFY_GROUNDSKILL packet{};

	packet.PacketType = HEADER_ZC_NOTIFY_GROUNDSKILL;
	packet.SKID = skill_id;
	packet.AID = bl.id;
	packet.level = skill_lv;
	packet.xPos = x;
	packet.yPos = y;
	packet.startTime = client_tick( tick );

	if (disguised(&bl)) {
		clif_send( &packet, sizeof( packet ), &bl, AREA_WOS );
		
		packet.AID = disguised_bl_id( bl.id );
		clif_send( &packet, sizeof( packet ), &bl, SELF );
	} else {
		clif_send( &packet, sizeof( packet ), &bl, AREA );
	}
}

why the packet.AID is changed to basically -bl.id ? for the disguised player, how does that effect the client? and why when i tried to just yet the code, i changed nothing ? (just keeping clif_send( &packet, sizeof( packet ), &bl, AREA );)