I suggest you seek out a mentor (either within your organisation or within Blue Prism) who can help you as this thread looks very confused to me.
1. Always use the Mainframe interface type (the one with the Grid overlayed over the mainframe screen) - not the Windows interface for mainframes UNLESS you have a very good reason not to (such as it being presented over Citrix).
2. For the Mainframe interface (which uses hllapi API), there is a Mainframe Automation Guide, and lots of articles in the knowledgebase. If your mainframe is not hllapi complient I suggest finding a hllapi complient mainframe instead of going down the Surface Automation route (it will be cheaper in the end - as SA=slow and more BP licenses, hllapi=very quick).
3. For your last comment Akos, you can activate any window in an operating system, irrelivant of the application type. You would need a Windows object for the mainframe, you would need to identify the entire window (not an element within it) using Win32 mode, and the Activate Application action in a mainframe stage will be available to you. However, if you are using a Mainframe interface the window does not need to be active. There is a guide all about Surface Automation of Mainframe Emulators that can be usd.