functional index condition

hello,

could you be so kind please and explain the condition in this functional index ?

CREATE UNIQUE INDEX "FSIPRDLIB"."HOUSEBILL_ACTIVE_UQ_IDX" ON "FSIPRDLIB"."HOUSEBILL" (CASE "STATUS" WHEN 4 THEN NULL WHEN 5 THEN NULL ELSE "WHCD" END , CASE "STATUS" WHEN 4 THEN NULL WHEN 5 THEN NULL ELSE "HBNUMBER" END )

PCTFREE 10 INITRANS 2 MAXTRANS 255 COMPUTE STATISTICS

STORAGE(INITIAL 65536 NEXT 1048576 MINEXTENTS 1 MAXEXTENTS 2147483645

PCTINCREASE 0 FREELISTS 1 FREELIST GROUPS 1

BUFFER_POOL DEFAULT FLASH_CACHE DEFAULT CELL_FLASH_CACHE DEFAULT)

TABLESPACE "FSI" ;