From legacy/trac#14742, we have special behaviour when a config option when a config option is passed on the commandline with a '+' or '/'... https://gitweb.torproject.org/stem.git/tree/test/integ/process.py#n199 This looks to presently be undocumented. Obvious guess is 'adds/removed from existing config rather than replacing' but I'm getting some unexpected behavior when I pass '/PublishServerDescriptor'. Probably only works properly for LineList arguments.