Path: Main Menu - Challenge - Computer - Customize
This menu allows you to select a custom engine and level. This will give you a more repeatable opponent for your training.
Those opponents do have an ELO rating, that is initially set to an educated guess.
At the moment, there are 4 different chess engines available. The performance of them varies pretty wildly, you cannot compare them based on level.
A list of engines, in increasing performance:
Angie is way stronger than all other 3 engines. Tania and Jean are somewhat comparable, making Rick a lot stronger than both of them.
You can edit the level at will.
When pressing start, you will get a confirmation dialog that also indicates current statistics of the selected engine, before the actual match starts.
For the actual engines used, all i can say at the moment is: the names are a hint, but some pretty far stretched.
All engines play 'standard' chess. However, some (fide, typical) rules are not enforced. The in-game board is responsible for rule validation.
Alternate engines are likely implemented in the future. At the moment there is a wide offering for all strengths of players.