[sf-lug] apt doc question 1

[jim]

regarding section 2.1 in 
http://www.debian.org/doc/manuals/apt-howto/ch-basico.en.html 

here's an example line from a typical /etc/apt/sources.list file: 
deb http://http.us.debian.org/debian stable main contrib non-free 

the line has space characters, which i presume are field delimiters. 
the text refers to the use of deb as the first token but is silent 
on the construction of the fields as well as the use of the 
subsequent tokens in the examples. 

first field: 
   examples show either deb or deb-src 

second field: 
   examples show URIs 

third field: 
   examples show stable/non-US and stable/updates and stable. 

fourth field: 
   examples show main only 

fifth field: 
   examples show contrib only 

sixth field: 
   examples show non-free only 

it would be very helpful if the text defines the purpose for each 
field (e.g. first field specifies binary or source, second field 
specifies a URI for a repository...). 

it would be helpful to know if any fields are optional. typically 
first fields are required and if any fields are optional they are 
those at the right. 

where to find answers to the implied questions re above?