What default is - SET behavior

I'm interesting in problem "Allow commenting of variables in postgresql.conf to restore them to defaults".  And I need
someclarify of SET command behavior.<br /><br /> Res_value is defined  in the source code as highest overriding setting
duringstartup (or reconfiguration) and it is used for store "default" value. Hovewer documentation of SET command (<a
class="moz-txt-link-freetext"
href="http://www.postgresql.org/docs/8.1/interactive/sql-set.html">http://www.postgresql.org/docs/8.1/interactive/sql-set.html</a>)
talkabout "<tt class="LITERAL"><span id="__firefox-findbar-search-id" style="padding: 0pt; background-color: yellow;
color:black; display: inline; font-size: inherit;">DEFAULT</span></tt> can be used to specify resetting the parameter
toits <span id="__firefox-findbar-search-id" style="padding: 0pt; background-color: yellow; color: black; display:
inline;font-size: inherit;">default</span> value." And there is question what is the meaning of "default value". 
BecauseI understood "default" like value coded in source code - mentioned in the postgresql.conf.  <br /><br /><br />
ThanksZdenek<br />