This mechanic is applied mainly in MMORPG games. What this is, is items that the player receives but they can not use them because they are on the demo version of the game or a free trial. This is a good way of encouraging players to buy the game as they will be able to use the items.
Runescape is a good example of a game that uses this mechanic. The game is free to play however players can pay money every month so that they have a subscription. While they are playing in free mode they will sometime receive items from quests or even from other players such as ‘Dragon Armour’. When the player tries to equip this armour it will tell them that they need to buy a membership to be able to use this mechanic. The player is then encouraged to buy a membership which then gives the company a profit.
Another good example of this mechanic being used is in the console version of Minecraft. Players can play through a demo of the game that only lasts a certain amount of time before it will ask them to buy the game or restart the demo. Since the player has already put time into the game creating their own buildings and gathering resources they will not want to lose it. Therefore they then buy the game so that they can keep on playing.