Credits are the number of SMS you currently have in your account. When you send an SMS, your credits get deducted accordingly. Credit Deduction refers to the number of credits deducted from your balance for sending the message.
When you send an SMS in English the credit limit is 150-160 characters whereas in the Unicode option i.e. when you send SMS in other languages, the limit is 55-70 characters.
The value for a single credit in English is 160 characters. With the number of characters increasing, the second credit consists of 306 characters and moving on in this manner the value of credit deduction increases by adding 153 characters in the SMS every time. As the number of characters keeps on increasing the number of credits deducted will also increase according to that.
For Unicode the credit calculation as below:
70 Characters (Standard SMS) 1 SMS Credit
134 Characters (Multi-part SMS) 2 SMS credit
(2 * 67 characters) 2 SMS Credits / Units-201 Characters (Multi-part SMS) 3 credits
(3 * 67 characters) 3 SMS Credits / Units-268 Characters (Multi-part SMS)
(4 * 67 characters) 4 SMS Credits / Units-335 Characters (Multi-part SMS)
(5 *67 characters) 5 SMS Credits / Units-402 Characters (Multi-part SMS)
(6 * 67 characters) 6 SMS Credits / Units-469 Characters (Multi-part SMS)
You can see the number of credits as well as a deduction in the delivery report of your SMS.