Android Layout - when to use app: vs android:?

You can use the app namespace to have app compatibility with older API versions.

For example app:srcCompat="@drawable/customborder" has the same effects with android:background="@drawable/customborder" The difference is that the first will work correctly with older API's and the second will not display what you would like.

You are talking about custom namespace.In android we can create custom views in additional to already available views. As per in Google developer docs.. To add a built-in View to your user interface, you specify it in an XML element and control its appearance and behavior with element attributes. Well-written custom views can also be added and styled via XML. To enable this behavior in your custom view, you must:

Define custom attributes for your view in a resource element Specify values for the attributes in your XML layout Retrieve attribute values at runtime Apply the retrieved attribute values to your view

Once you define the custom attributes, you can use them in layout XML files just like built-in attributes. The only difference is that your custom attributes belong to a different namespace. Instead of belonging to the namespace, they belong to[your package name]

So for if you use default views you can use android namespace and if you want to set and use attributes for custom view you can define your own name.

Refer this