Your current balance is the amount currently owing on your card account. The available credit is the amount that you have available to spend. This is based on the credit limit less the current balance less any pending transactions. The credit limit is the amount of credit available on your card account. You can spend up to this limit. If your account has a credit current balance or the pending transaction is a payment, the available credit will be your credit limit plus the credit current balance plus the credit pending transaction.
Note that 1 milliliter is equal to 1 cc. There are about 20 drops of water in 1 milliliter. Eye drops are about the same. For other liquids, the number depends on the viscosity of the liquid, which affects the average drop size. Thicker, more viscous fluids are likely to produce larger drops and therefore fewer drops per milliliter. Note that 1 milliliter is equal to 1 cc. There are about 20 drops of water in 1 milliliter.
I have tested a few different droppers and transfer pipettes (putting a fixed number of drops into a small grad. cylinder and using the raw value to find a per ml equiv.) and it seems like the general range is 20-30 drops per ml, so I see why 20 drops is a reasonable estimate - for some reason e-liquid-recipes defaults to 35, and I don't think I have ever seen a dropper that is that precise - only pipettes.