How are loans allocated among clients when the supply of shares available to lend exceeds the borrow demand?

When you sell stock short, IBKR must locate and borrow shares to make delivery to the purchaser at settlement. Specific cost to borrow those shares depends upon the supply of shares available to borrow and the demand from other short sellers to borrow shares…

ACCESS SHORT SECURITIES (SLB) AVAILABILITY TOOL

When you short sell a stock, IBKR must locate and borrow shares to make delivery to the purchaser at settlement. Whether or not there is a cost to borrow those shares depends upon the supply of shares available to borrow and the demand from other short sellers to borrow shares…

In the event that the demand for borrowing a given security is less than the supply of shares available to lend from participants in the Stock Yield Enhancement Program, loans will be allocated on a pro rata basis. 

Example…

No. Stock Yield Enhancement Program loans can be made in any whole share amount although externally we only lend in multiples of 100 shares. Thus the possibility exists that we would lend 75 shares from one client and 25 from another should there be external demand to borrow 100 shares…