A better way might be to say that:
Sleep mode is application instance specific, or, in technical terms, process specific. There are programs that use one instance to display multiple windows, and there are programs that tie windows to instances of themselves. The former is the case with Edge, and the latter is the case with Notepad.
NVDA's own user guide states that sleep mode is active for the current application, which also is incomplete based on the logic used here. I would imagine it was stated like this with the assumption that one app meant one window and to simplify the concept of "processes" for users. Perhaps I didn't use an easy to understand terms in my previous message...
Speaking of processes, an overview for users: a process is an instance of a program (or an application) coming to life i.e. being run by a computer user. A process consists of an identifier commonly known as a process ID (PID for short), the actual code and data of the program itself, a collection of metadata about the program such as resources used by the program, and at least one thread (or thread of execution) responsible for running the program code. When you run a program, the operating system (in this case, Windows) will load the program into memory, create PID and other metadata, and create a thread to run the program code (at least these are the minimum steps to starting a program; it is more complicated than this but at least I hope you understand the idea).
It is perfectly possible for programs to specify whether they wish to run a single instance of itself for everything, or run multiple instances of themselves. In case of the former, only one process can represent the program, or the program can ask the operating system to run different programs for specific tasks (for example, specialized tasks within web browsers, and using a specialized program to allow NVDA to talk to 64-bit programs more effectively). In case of the latter, each instance of the program (application) is a different process, as demonstrated in this thread. To summarize, a process is a program (application/app) stored on disk coming to life when you run it (opening it from Start menu, clicking a file type associated with the program, etc.).
Hoipe this helps.