I'm getting ready to turn on the ability to leave comments on my blog, but I'm not sure of the difference between the two options.

The MT Manual Page for the Preferences section doesn't seem to cover several of these items, nor does the comments documentation.

Any insight into this would be appreciated.

Thank you.

All this means is that if you leave it on OPEN, then each time you create a new entry, the default status of comments will be open (meaning people will be allowed to leave comments).