What is “top-level partitioning” in architecture?

I am reading the book Fundamentals of Software Architecture . And I keep seeing this word “top-level partitioning” in a chapter. What does “top-level” mean here? Does it mean the best or the most advanced?

Architecture Partitioning

The First Law of Software Architecture
states that everything in software is a trade-off, including how
architects create components in an architecture. Because components
represent a general containership mechanism, an architect can build
any type of partitioning they want. Several common styles exist, with
different sets of trade-offs. We discuss architecture styles in depth
in Part II. Here we discuss an important aspect of styles, the
top-level partitioning in an architecture.

An excerpt of the chapter is captured as an image here