I can't find any useful explanation by googling, and I don't see anything in the IMDB database files.
What is the meaning of nr_order in cast_info?
TIA
It is a number to indicate the billing position in the credits.
It's defined here in the db schema.
This shows where the value will be filled up.
Looking in the actresses.list.gz file I found this at the top:
"xxxxx" = a television series
"xxxxx" (mini) = a television mini-series
[xxxxx] = character name
<xx> = number to indicate billing position in credits
(TV) = TV movie, or made for cable movie
(V) = made for video movie (this category does NOT include TV
episodes repackaged for video, guest appearances in
variety/comedy specials released on video, or
self-help/physical fitness videos)
Looks like there wasn't any useful documentation for it in IMDbPY either.