Not sure if this is related to the difference between the "Non-Lendable" and "Lendable" fund:

https://www.blackrock.com/institutions/en-zz/solutions/securities-lending

Edit:

Also found this (different from the fund I'm curious about but I'm guessing this is what a "Lendable" fund means):

https://www.blackrock.com/us/individual/literature/summary-prospectus/sumpro-lpi2060-p-us.pdf

The Fund and certain Underlying Funds may also lend securities with a value up to 33 1⁄3% of their respective total assets to financial institutions that provide cash or securities issued or guaranteed by the U.S. Government as collateral.

So I'm guessing it is about lending assests in the fund to other financial institutions.

Not sure if this is related to the difference between the "Non-Lendable" and "Lendable" fund:

https://www.blackrock.com/institutions/en-zz/solutions/securities-lending