Unused entries in code book

Hi,

I am trying to understand the building of Huffman codes from the code 
lengths. In the Tremor code first I see that the codewords are being 
generated by the function _make_words() and then sorted.

After this I see some magic code and something related to unused entries. 
Does the code generate code words for unused entries too? Are these unused 
entry code words used during the decode process? If these are used why do we 
call them unused entries?

Sorry if my queries are silly, but I really need to understand them. Thanks 
in advance for your help.

-- 
P.Vamsi Mohan Reddy,
Hyderabad.

Bad leaders are elected by good citizens who do not vote.