Lines that are part of a center environment will be displayed centred if the destination format centers text.
You can use the center environment inside other environments. You can also use it inside another center environment, even if this may be senseless.
If you use other environments inside a center environment they will be layouted like in all other cases. Only when the center environment is the active one text will be printed centered.
If the following example isn't centered the current documentation format doesn't allow it to use centered text.
!begin_center A centered line. A centered paragraph with two source lines. The Guide to (!nl) UDO !end_center
You see that UDO layouts paragraphs of a center environment, too. To insert a manual line break use the (!nl) command.