If the region is active, then many Magit commands act on the things that are selected using a mechanism based on the region instead of one single thing. When the region is not active, then these commands act on the thing at point or read a single thing to act on. This is described in the previous section — this section only covers how multiple things are selected, how that is visualized, and how certain commands behave when that is the case.
Magit’s mechanism for selecting multiple things, or rather sections that represent these things, is based on the Emacs region, but the area that Magit considers to be selected is typically larger than the region and additional restrictions apply.
Magit makes a distinction between a region that qualifies as forming a valid Magit selection and a region that does not. If the region does not qualify, then it is displayed as it is in other Emacs buffers. If the region does qualify as a Magit selection, then the selection is always visualized, while the region itself is only visualized if it begins and ends on the same line.
For a region to qualify as a Magit selection, it must begin in the heading of one section and end in the heading of a sibling section. Note that if the end of the region is at the very beginning of section heading (i.e. at the very beginning of a line) then that section is considered to be inside the selection.
This is not consistent with how the region is normally treated in Emacs — if the region ends at the beginning of a line, then that line is outside the region. Due to how Magit visualizes the selection, it should be obvious that this difference exists.
Not every command acts on every valid selection. Some commands do not even consider the location of point, others may act on the section at point but not support acting on the selection, and even commands that do support the selection of course only do so if it selects things that they can act on.
This is the main reason why the selection must include the section at point. Even if a selection exists, the invoked command may disregard it, in which case it may act on the current section only. It is much safer to only act on the current section but not the other selected sections than it is to act on the current section instead of the selected sections. The latter would be much more surprising and if the current section always is part of the selection, then that cannot happen.
This variable controls whether the region is visualized as usual even when a valid Magit selection or a hunk-internal region exists. See the doc-string for more information.