Could someone tell me what this statement is doing?

AND CASE WHEN pcl.clec_dspch_type_cd = ''ZZZZZZ''
THEN ''ZZZZZZ''
ELSE DECODE(SUBSTR(pcl.clec_dspch_type_cd, 4, 1), NULL, SUBSTR(dm.dspch_type_cd, 1, 3)
, DECODE(dm.prod_id, 1, ''ZZZZZZ'', dm.dspch_type_cd))
END = pcl.clec_dspch_type_cd