When sending out your message, you have the option to choose between "TEXT" or "UNICODE" message encoding.
With TEXT encoding, you can use all the most common characters in the alphabet.
With UNICODE encoding, you can use special characters, like chinese, arabic, emoticons, ... The downside of UNICODE is that your message is limited to 70 characters per SMS, or 63 characters per SMS if you are sending a message that spans multiple parts.
Practical example: you send out an SMS of 120 characters.
TEXT encoding: you are invoiced 1 SMS
UNICODE encoding: you are invoiced 2 SMS
We generally recommend to use the TEXT encoding, since this more cost efficient and should be enough to cover the need for most of your messages.
For more details on what characters are in the TEXT encoding: https://support.ringring.be/hc/en-us/articles/115005250203-What-characters-can-I-use-in-a-text-message-