The type of cel doesn't change based on its 
quality, size, or detail. A cel of half a body in 
Brain Powered and a cel of San's front facing 
face in Princess Mononoke are both production 
cels.

Conversely, a NP cel made by the studio can be 
high quality or low, but it isn't classified as 
prod/pre-prod artwork even if the quality is 
high. 

Btw, making NP gift cels seems to be a relatively 
common practice, although I hear about it more as 
gifts to studio employees.