Do NOT use it to craft any of these pieces: bracers, cloak, belt or boots.
It’s a noob trap, i repeat, stay away! and do NOT craft them.
Since it appears that only the main pieces will give you the set bonuses…
At least, that’s what the wording implies…
So for example if you have tier chest, helm and gloves… make some pants or shoulders to get 4 piece.
That perfectly good pair of 278 ilvl boots you happen to have… nope, you’ll have to make the 264 pants into tier and waste a charge of the catalyst if you get higher ilvl pants soon™.
Because if it worked on any of the mentioned slots giving the set bonus, you could have made the bis boots your 4th piece.
Now you’ll have to use an inferior ilvl main piece and then use another charge later if you happen to get a higher ilvl in that slot.
Hope i explained it well enough, good luck and use your catalyst charges carefully!