End mills are round; therefore inside corners naturally carry a minimum radius related to tool diameter. Specifying a zero-radius internal corner often forces EDM, corner drilling, or unrealistic assumptions.
Increase internal radii slightly beyond the tool radius when you can—it improves tool life and surface finish in pockets.
For parts that must mate with square-edge components, document whether a small corner relief or a controlled chamfer is acceptable at the interface.
Thinking in terms of reachable geometry early avoids expensive secondary operations disguised as “simple” edits late in the program.