Have a screenshot, and below it, a brief explanation. 
Currency, found throughout the Tower, is in part being used to replace some older under performing or troubled systems. Two such systems are the “training point” system used to transfer abilities between demons and the “ability teaching” system used by the player’s main character to learn abilities from demons.
These systems were good first passes, but time and feedback had exposed some flaws in them: training points tended to accumulate both too slowly and too quickly. 
Players often felt obligated to keep demons around for a significant period of time just to transfer their abilities around, and yet also, well-used demons tended to end up with tons of excess training points that could be used to game the system by teaching an ability to them from a demon with only 1 point, then using their much larger pool to spread it around as desired
The new system of simply buying training directly with currency is much more straightforward. Generally speaking, you have much more control over ability copying now (since it is no longer tied to individual demons’ training point pools)… but counterbalancing that are the facts that currency is a shared resource with other systems, and that training has been priced such that the total number of ability copies available will be less than before. Fewer, but better, ability copies, is the outcome.
You may notice the main character has a training cost listed as well: direct purchase of training has also replaced the “ability teaching” system. 
However, one old rule still applies: while the main character can be taught by demons, they cannot teach demons. This allows starting abilities and abilities learned by defeating heroes to remain semi-restricted and powerful, since they still cannot be immediately copied around to an entire party just because the main character learned them.
More details about these systems, as well as information about the other things currency will be useful for, will be coming soon. Cheers!
