Allow for WP/plugins: What is this feature?

First of all, the CustomPress plugin is a great feature. Very much needed. A great start.

I was wondering, what is meant by “Allow for WP/plugins (can’t be changed)”? What is meant by the “WP”? Does this mean that plugins and WordPress features can’t access the Custom Field? Not sure what the ramifications of that is quite yet, so not sure if I want to deny it or now.