Template:M summ Equity Derivatives 12.9(a)(vii)-(iv)

Loss of Stock Borrow is an Additional Disruption Event in the 2002 ISDA Equity Derivatives Definitions, and is fondly abbreviated, by this commentator at least, to LOSB. It pairs nicely with an Increased Cost of Stock Borrow, fish or chicken. See also 12.9(b)(vii) which deals with the tension between LOSB and Hedging Disruption.

• Where the Hedging Party can’t locate a stock borrow, the Non-Hedging Party has the option to source one that is struck at less than the Maximum Stock Loan Rate within two Scheduled Trading Days, failing which the Hedging Party can terminate the Transaction.
• Where LOSB and Hedging Disruption both apply and the same event could qualify as either, it will be treated as a LOSB (which has milder consequences for the affected party).

Loss of Stock Borrow under Synthetic PB: For synthetic prime brokerage, it is common for the PB to pass on its stock borrowing costs (well: it is a synthetic equivalent of a stock borrow and a short sale, after all, so this makes sense). It does this by subtracting the prevailing borrow rate from the floating rate it pays under the swap. Therefore the Non-Hedging Party wears the ultimate cost of the expensive stock borrow, so there’s no real need to impose a Maximum Stock Loan Rate (though prime brokers will typically impose one as a matter of course).
Comparing Loss of Stock Borrow and Increased Cost of Stock Borrow: There is a logical hand-off and interaction between Loss of Stock Borrow with Increased Cost of Stock Borrow:

• Under a Loss of Stock Borrow the Non-Hedging Party has a bit less flexibility in what it does: it must pony up (or procure) a stock borrow within 2 Scheduled Trading Days itself, or Hedging Party can terminate outright. Under Increased Cost of Stock Borrow, the worst that can happen is the trade is repriced to take in the higher rate. So ICOSB is the “gentler” provision from the Non-Hedging Party’s perspective.
• If the cost of a stock borrow exceeds the Maximum Stock Loan Rate it is deemed to be (as good as) impossible to borrow stock, so it is treated as a Loss of Stock Borrow, not merely an Increased Cost of Stock Borrow.
• If a counterparty wants to apply Increased Cost of Stock Borrow whatever the cost of an available bid — and given that it can pass the cost on, a synthetic prime broker might be happy to do this — the answer is to disapply Maximum Stock Loan Rate altogether. This means that any possible stock borrow rate, however astronomical, comes under Increased Cost of Stock Borrow, and Loss of Stock Borrow (which is slightly more onerous a termination right) only applies where there are no offers in the market at all.