'man ps': What is the difference between 'CODE' and 'HEADER'?

[Detlef Lechner]

Hello,
'man ps' in the section "STANDARD FORMAT SPECIFIERS" lists a table 
headlined "CODE HEADER DESCRIPTION" to control the output format of the 
ps command. When do I have to use 'CODE', and when do I have to use 
'HEADER'?
What is the difference between the two?

Regards,
Detlef Lechner