SMS Credit

An SMS Credit is an SMS message to one mobile number up to 160 characters. Multiple credits can be combined to create a longer message to recipients.

1 credit = up to 160 characters
2 credits = up to 306 characters
3 credits = up to 459 characters
4 credits = up to 612 characters

These restrictions are put in place by the networks and relate to something known as concatenation. When you have multiple messages passing through the networks they are actually broken down as individual SMS then 'knitted' back together when they hit the mobile phone. Your mobile phone knows in what order to put the content by invisibly utilising 7 characters of each SMS. As such, when you are sending 2 x long SMS, 14 characters are utilised. When sending 3 x long SMS, 21 characters are utilised etc.

References

Wikipedia