Setting a Condition in Vbulletin

[RVSmarter]

I have a conditional statement as follows:

<if condition="$show['custom']"> <img class="inlineimg" src="$stylevar[imgdir_misc]/custom.gif" alt="$vbphrase[custom]" /> </if>

Where and how do I set the condition "$show['custom']" ?

Do I just put the code

$show['custom'] somewhere in the post that this is a condition for?

is there a Set condition command?

Thank you for the help.