Each history file contains it's internal name, much like a file system has an inode # that is the internal name for that file. We call the names "keys" and you can dig out the inode key like so (every delta has a key, the first delta is called the rootkey):

    $ bk log -nd:ROOTKEY: -r+ slib.c
    lm@bitmover.com|src/slib.c|19970518232928|52808|f3733b2c327712e5

The key is user@host|relative path|UTC|checksum|64 bits of /dev/random and they are guaranteed to be unique if your DNS config is correct (we don't create duplicate keys, look for the uniq db in the code).

The way we version the entire tree is simple, it's logically (implemented differently for performance):

    <rootkey> <delta key>
    <rootkey> <delta key>
    ...

where each rootkey uniquely identifies a file and each delta key identifies the tip as of that commit.

Not sure if that is clear enough or not, ask away if not.