incoming_files is for all single file downloads, the directory is shared recursively (changed in 2.8.2 due to user_commit_dir option)
incoming_directories is for all multi-file downloads (from bittorrent), the directory is shared non-recursive
all_files is for other dirs than ./incoming, the directory is shared recursively
only_directory is for other dirs than ./incoming, the directory is shared non-recursively
Starting with CVS 2.5.29 you have to have at least two shared dir entries in downloads.ini, one with
strategy incoming_files and one with strategy incoming_directories,
otherwise the core will create default directories. Those two entries can point to the same directory.