CBase Chess for the iPad/iPhone and compatible iOS devices shines in support of variations. There are a small number of mobile apps that support study of games with variations, but no other app that I have used handles them as well as CBase Chess. When a point is reached in the game where there is a variation in the annotations, a pop-up appears beside the board. This unobtrusive notification is as it should be. It is possible to continue studying the position, and considering each of the candidate moves, before selecting one for further exploration.
Strangely, most mobile apps place the pop-up over the board.
The app is quite limited in features. It is designed for one purpose: a PGN viewer. The user uploads PGN (portable game notation) files to the program, where it is possible to play through the games, and through the variations. The app supports both text commentary and variations, including layers of sub-variations.
I seek a chess app that will permit me to view the exceptional annotated games published in Chess Informant. CBase Chess is the best that I have found. Unfortunately, it does not appear to support Informant codes. Loading Karpov's Golden Games from the Best of Chess Informant CD, made it possible to play through Karpov's games with Grandmaster commentary in the form of variations, but did not leave in place the commentary embedded in Informant's codes (see "A Fingerprint"). To be fair, some of these codes do not translate well in PGN format, but require Informant's proprietary format.
CBase Chess rotates easily between landscape and portrait.
There are features that would be desirable: user choice of chess set and colors, a built in engine, and a feature for adding commentary. I suspect that developer NM Austen Green has these on his to-do list. The app reveals the signature of a developer who seeks tools for chess improvement, rather than someone driven by commercial ambitions. The app is free and does not carry advertising.
I have more than thirty chess apps on my iPad. CBase Chess is one of a few that I use frequently.