@grgarside Table 74 at developer.apple.com/library/mac/documentation/Networking/… gives context to the AFP replay cache, a feature of AFP 3.3 that was introduced with Mac OS X 10.6.

Side note: somewhere, maybe not in Stack Exchange, there's a question about the operating system and file system within e.g. Time Capsule – I don't recall a conclusive/definitive answer to that question.

@grgarside if the dialogue is novel, pre-release, then we can search the pre-release space at Apple Developer Forums and search/browse asciiwwdc.com

I can't find anything relevant in Apple's space.

Recommended reading: What’s New in Foundation Networking (WWDC 2013 session 705) and Best Practices for Progress Reporting (WWDC 2015 session 232).

Whilst neither session mentions interrupt or partial, clearly it's desirable to resume a copy (or the copy part of a move operation) wherever a file is found to be partial as a result of interruption to a network connection.

@grgarside with attention to the exact wording in that dialogue:

! A partial copy of the folder "Spigot" has been detected. Do you want to resume the copy? … Skip … Stop … Resume

Can you reproduce the prompt to resume, with a partial copy of a file that's not a directory file (not a folder)?