I just know this ^ 
Since this is a client script so 
function replace () 
outputChatBox ( "> replacing the euros vehicle" ) 
  
txd = engineLoadTXD ( "data/euros.txd" ) 
engineImportTXD ( txd, 587 ) 
dff = engineLoadDFF ( "data/euros.dff", 587 ) 
engineReplaceModel ( dff, 587 ) 
addEventHandler ( "onClientResourceStart", replace ) 
 
but data means inside the data folder in the same directory of the script, right? 
Just like in html?