The CBH format, existing since ChessBase 6.0, reached its limits in terms of functionality. The current *.2CBH format, newly introduced with CB17, removes these limits. There are e.g. no limits for player names, tournament identifiers etc. anymore.
The "2CBH" format gets along with less files and uses modern caching methods of the operating system.
Furthermore, it no longer needs search accelerators, whose update always brought comprehension and performance problems.
2CBH is conceptually simpler and thus more robust. Functions such as reorganizing therefore sometimes run twice as fast as in the old CBH format. The new format needs more space on the disk, which is relatively unimportant with today's disk sizes.
With the new format comes new functionality. First, CB17 brings an identifier for the end of a game. After the end of the game (result), moves can still follow. Another thing is the determination of the beauty of games. The beauty value is written in the new format in the game identifiers, in the old format there was no space left.
Many of the new functions introduced with CB 17 are only feasible with the new 2CBH format. The introduction of a new data format is always seen with somewhat mixed feelings by many users, we are aware of that.
The present program version is completely downward compatible and you can continue to use all previously supported data formats such as CBF,CBH,PGN,etc. for the purpose of forwarding, data backups, etc..