Most probably it means that if you only use 8kb, let's say u sent a text via whatsapp and then you stop using the phone; no data in or out, then you will be charge for that session which is 5 sen.

Cos your session is just 8Kb for example. Per session; in that session, you only use or text once. Otherwise you continue to use until or before you reach 1 MB, but how are you going to monitor your usage and also maybe that time you have no need to use or to send message to anyone.