how to launch a datafield in a device app ?

Nearly beginner: I have written a watch face already and want to write an app now with a datafield (never done it). 

If I write an iq app, how do I access a datafield in the app? The situation I imagine is that I have two independent applications on the watch: the app and the datafield I want to access by the app. 

(I guess if you have an independent datafield, written by yourself, on the watch, you can use it in a native app.So my question is again, can you use such a datafield in an iq app too, or do you have the datafield to be embedded in the app itself? And how would you do that?)