"Receivable" means receivable now or in the future.
Basically, SCSB is the tax-free amount of her redundancy payment but is reduced by the value of any future tax-free lump sums payable by a pension scheme. She can waive her rights to the pension scheme lump sum and this will increase the SCSB amount by the same. It's a trade off between a higher tax-free lump now or a getting a tax-free lump sum on retirement. Obviously, depending on the scheme benefits, the future lump sum is a known or unknown quantity ie is it fixed or will it grow over the years ?