The custom Stereotype keyword «M:Package» indicates a Wolfram Language package, where each «M:Package» is a customised (domain specific extension) SysML Block, and can thus carry Activities and OpaqueBehaviors corresponding to functions, and other SysML representations of aspects of a Wolfram Language package.
Note that an «M:Package» does not carry Operations (although a SysML Block can).
In this trail section we'll look very briefly at some overviews of how the Webel W`Base` packages depend on each other and the low-level Devel`, Utility`, and Display` packages, which should be obvious to most Mathematica users, and the very handy ManGUI` package utilities for Manipulate GUIs.
Most of the other packages have dedicated sections in this trail.