Which is the criteria to show in the game log that you purchased an item? [duplicate]

Possible Duplicate:
What items are announced to chat when bought?

Please, close it as duplicate. Thanks!

When someone purchases the item Frozen Heart (by example), in the game log, where we chat, a message appears like "Player ETC get a Frozen Heart" (I don't know the exact message because I'm using the pt-br version). What's special about that and another items, to be revealed in the message log with that message? I mean, Frozen Heart costs 2775g (650g), while Trinity Force by example costs 4070g (300g), it's a hard to get, expensive and phenomenal item, but messages about it don't appear when you purchase it in the shop.

Which is the criteria that game creators used to show that message then?

I suspect that purchases are printed to the game log if they have some team-wide (possibly unique) effect, so that multiple teammates know not to rush the same items. For example, Frozen Heart has a unique aura; if one teammate has one, the benefit of a second teammate buying one would be significantly reduced. So in that case, the game warns you of the purchase. Trinity Force only really affects the player who bought it, so it doesn't really matter if your team has multiple copies.

Another example would be wards (which are printed when purchased). Wards aren't unique, but they provide a temporary shared benefit to the team; you don't necessary want the whole team stocking up on wards at the same time.