Select geometry connected to already selected elements.
This is often useful when a mesh has disconnected, overlapping parts,
where isolating it any other way would be tedious.
To give more control, you can also enable delimiters in the Adjust Last Operation panel,
so the selection is constrained by seams, sharp edges, materials or UV islands.
With Pick Linked you can also select connected geometry directly under the cursor,
using the L shortcut to select or Shift-L to deselect linked.
This works differently in that it uses the geometry under the cursor instead of the existing selection.
Select connected faces with a reference angle less than the value specified.
Looking at the image above, when at least one face is selected (as seen on the left),
Linked Flat Faces will select all connecting faces that lie
on the same or similar plane (as shown in the middle image).
If the corners are smoothed, those faces are no longer lined up with the selected faces.
At this point, increasing the Sharpness value in the tool options could include the smoothed faces.