One SMS message is limited to 160 characters, which we count as one message segment.
Messages that are longer than 160 characters are divided into groups of messages of 160 characters each. If you send a message with 310 characters, this would count as 2 message segments, as it will take two 160 character messages to transmit.
If you include unicode characters or any special characters like emoji or new lines in your message, this requires us to send additional data with each message so cell phones can render them properly.
Unicode characters on message segments
Since that additional data takes up space in the 160 character limit, messages with unicode characters need to be 70 characters or less in order to count as ONE message segment. If you send a message that is 160 characters with one or more unicode characters, this will count as 3 message segments, as it would require THREE 70 character messages to transmit.