w w (magit-am-apply-patches)
This command applies one or more patches. If the region marks files, then those are applied as patches. [...]
This raises the question:
How do I use the region to select the patch files to be applied? In particular, the region of which buffer is intended to be used for that?
Looking into the sources, I found that the command
magit-am-apply-patches uses the function
magit-region-values to evaluate the selection. The latter's description explains that the region should constitute a "valid section selection" for this and refers to the description of the function
magit-region-sections for details. That in turn states:
When the region is active and constitutes a valid section selection, [...]. This is the case when the region begins in the heading of a section and ends in the heading of a sibling of that first section.
To profit from this description, one should know:
Which sections in which buffer show plain patch files that lie anywhere in the local filesystem?