Well to help clear things up here's a quote from a thread posted by Mod Mark on the Runescape forums. (Quick Find code = 16-17-315-56584750) Realize this was dated May 16th and some changes may have been made.
Quote:
1. Only certain specific items will be lendable. We can't allow people to borrow items that are untradeable, that degrade during use, that have charges or get used up. We've focused on high-level combat kit such as Third-Age clothing, Waterbirth Island armour, rings, godswords, dragon weapons and armour, most rune stuff, mystic armour and dragonhide. These will allow you to help friends who are fighting large creatures with you. More items can be made lendable in the future.
2. Players may only lend out one item at a time, and borrow only one item at a time. We accept that this is a pretty tight restriction, but there are reasons for it. In particular, people have raised concerns that there will be no incentive to buy items if they can be borrowed easily with a guarantee that they'll be returned afterwards.
3. When you lend an item, you'll be able to choose whether you wish to lend it for two hours, for a day, or just until log-out. Items lent out for a specific duration cannot be retrieved until the end of the loan period (due to technical restrictions), whereas items lent out for the rest of the login session can be demanded back at any time. This is a bit of a change from what we said before; thanks must go to the players who suggested it.
4. Borrowed items are protected over ALL other items if the borrower dies, and cannot be taken into Bounty Hunter. So, if you whimsically decide to borrow someone's santa hat to wear during a KQ trip, you're risking the loss of your own valuables and not the hat.
5. After some discussion, we decided to allow the lending of party hats, santa hats and Hallowe'en masks; before the trade restrictions came into effect, people frequently lent these items to their friends, so we're going to allow it with the item lending system too. I appreciate that people may feel this detracts from the glamour of showing off their valuable possessions, but people have been lending out those discontinued holiday items throughout the history of RuneScape, so we're going to permit it again.