I believe it means that a cel has an additional 
layer for an inanimate object as a part of the 
foreground. It is like a background in that it  
doesn't change/move like the characters in the 
inbetween layers do. 

The cel below is a book cel. Its not an easy one 
to see from the scan, but its the only example I 
own. If you look closely at the reflection of the 
green female in the water you can see that the 
plants at the edge of the lake are on top of her. 
Although, the plants are part of the "background" 
and don't change, their postion has to place them 
above a moving character. The female character is 
sandwiched between two background elements.  The 
order of the layers is 1) background 2) 
carriage+reflection 3) carriage door= reflection 
4) green female (Caroline)+reflection 5) second 
background element and 6) Masila with horse.

Sorry for being so wordy, I'm not very good at 
descriptions sometimes.