See Mike Harvey's viewpoint in Hotspot 14:
" Scales on other than
the dorsal ridge and lateral line are distracting.
Koi with such scales are called either
"Yoroi (armour)-goi" , "Yoroi Doitsu", or
"Ishigaki-rin" ( stone wall scales). They are
valueless, even if their colours and patterns
are beautiful.
"There are two types of Doitsugoi"
- the leather carp which has no scales and
the mirror carp which usually has large scales
along both sides of the dorsal fin and lateral
line. The latter is the original Doitsu-goi. The
moment that the scales are larger than the
normal wagoi the koi is effectively " doitsugoi"
and if the scales are on areas other than
the dorsal ridge and lateral line then they will
be "Yoroi Doitsu"
http://www.koi-clubs.com/southeast/HOT/Hotspot14.pdf