[bigbes]

bps-tree means B+-tree: unique combination of B+ and B tree. You can read wiki or "The Ubiquitous B-Tree" whitepaper https://wwwold.cs.umd.edu/class/fall2002/cmsc818s/Readings/b... .

You may read code of it here: https://github.com/tarantool/tarantool/blob/1.6/src/lib/sala... . It's very thoroughly commented.