Meaning of ${|} define\variable

That little define is made available thanks to the LogicLib.nsh header file.

You will usually see this used along side the ${IfThen} define.

Here are some examples:

;= This will only execute the !echo instruction if there were errors
${IfThen} ${Errors} ${|} !echo "This will only show if there are errors!" ${|}

;= This will check to see if the process "MyProcess.exe" is running.
;= If it is running then it'll terminate it.
${IfThen} ${ProcessExists} MyProcess.exe ${|} ${TerminateProcess} MyProcess.exe $0 ${|}

;= This one will check the host PC if it's running Windows XP or later.
;= If the check fails, it'll show a message box with the error message.
${IfNotThen} ${AtLeastWinXP} ${|} MessageBox MB_OK "ERROR: You need WinXP or greater to continue.." ${|}

So you can see that ${|} is just a means for the ${IfThen} to end it's conditional instructions.

You can always search the LogicLib's source code to see for yourself.

Hope this helps!