A “segment” refers to a part of a text message containing a maximum number of characters. Depending on the contents of the text message, or if the message is sent out in multiple parts, the maximum number of characters is different per segment.
If your message is over 160 characters, the message is broken down into multiple parts before being dispatched to the recipient's phone. Today, the majority of cell phone carriers join the multiple message parts to create a single message for the recipient to read. However, some phone carriers may send the messages separately, one after the other.
Here are some examples of why the character count per segment differs. Note that each segment includes spaces, punctuation, and numbers.
160 characters: This segment is for standard, single text messages that do not contain any special characters or are not part of a set of multiple text messages.
70 characters: This segment is part of a group of text messages sent as a set. For example, in a group of two segments, the first message will start off with (1/2) followed by the second part (2/2).
Curious as to why 160 is the magic number for text messages? Read more.