If I understand your question, you'd like a breakdown of the data usage on your phone based on application.
Go into Settings and Select Cellular Data. You can scroll down and see what apps are using cellular data, and you can disable certain apps from using Cellular, i.e. they won't exchange data unless on WiFi.
I'm glad to see that you're monitoring your data usage, wapainting. The Data Utilization tool will let you know general categories as to where your data is being used. Your iPhone will show specific app data usage as shown by stef7. You can reset your statistics in Settings>Cellular at the beginning of your new bill cycle if you would like to keep track of data usage by each specific application on your phone.
With regards to Data Utilization, you may hover your cursor over the "?" next to each category to get a better idea of what you may be using that is going towards this usage as outlined in #1 under Understanding the Data Utilization Categories here: http://vz.to/2iITTn0 .
The chat category will encompass any app used to communicate with another user through a chat function. A couple popular apps that are used to chat are Facebook Messenger and Skype. The audio category is data used for apps and websites used to download or stream music, radio shows, or podcasts. Some examples of these apps and websites are Pandora, Spotify, iHeartRadio, Apple Music, and iTunes. Does this help shed some light on our usage?
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Yes. iMessages uses data and it probably is considered chatting.
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