In general, the priority for which card to charge is: Pet Damage, Item Rating, Pet Rank, Residue, Milk. The notable exception being Reincarnation which you
want to charge once at the end of your ascencion to get over the reincarnation requirements. You can easily tell this by visiting the /cards page.
A
This is the current bonus given by the card. It is calculated based on your card level, the card's temporary power and the card's permanent power.
B
This is the card's temporary power and upon ascending it is reset to 0. It is gained whenever an expedition finishes and this card is selected from the pool of possible 3
for the card reward. This can be increased from 1 to 2 and 2 to 3 with later upgrades. Meaning later on, you can get all 3 cards from an
expedition upon completion.
C
This is the card's permanent power and it is kept across ascensions. You gain this by using up a charge to convert some of the card's temporary
power into the permanent power. When you have low amount of temporary power this can lead to having a decrease in overall benefit until temporary
power recovers from the charge - it is recommended you avoid charging if it is negative.