What exactly does the --dir (preopened directory) flag do? How does it differ from --mapdir?

[artlogic]

The subject says it all. I'm trying to understand what --dir does, and why it exists? How does it work with --mapdir? I've seen some odd behavior when using these with the WASI SDK and I'd like to understand what --dir's intention is. Thanks1

[syrusakbary]

--dir makes accessible certain filesystem directory to the program. Without it, the WASI program is not able to access the outside world (that means, that the programs are sandboxed by default)

[artlogic]

--dir makes accessible certain filesystem directory to the program. Without it, the WASI program is not able to access the outside world (that means, that the programs are sandboxed by default)

How does that differ from --mapdir?