[Mp4-tech] NAL unit type 12

[kaustubh.patankar vsnl.net]

Dear All
Will any one explain the practical use of NAL unit type 12 case?
The syntax defines it to be filler data. 
The case of filler unit can be in stuffing byte for under utilization 
of bit rate. Few encoders are implementing these bytes (filling with 
zero) as trailing zeros?.
Or can we put these zeros by making the NAL unit type as 12.
Please suggest.
with regards
Kaustubh