XSSI printenv directive
XSSI variables are set in a couple of different ways. You can set your own with the set command, or you can use one of the variables that is set automatically for each browser when it requests a page. To see the list for a specific browser, you can use the printenv command, which outputs a list of what are called environment variables for your specific case. The output of printenv looks like this:

HTTP_USER_AGENT = Mozilla/4.0 [en] (WinNT; I)

The first part (in uppercase) is the name of the variable. The second part (after the equals sign) is its value.

Basic syntax:
<!--#printenv -->

How to insert a new printenv directive
To add a printenv directive at the cursor's current location in your document, either choose "XSSI printenv" from the Insert menu or click the icon in the XSSI category on the floating Object palette. If you have automatic translation enabled (or you manually translate your document), the appropriate value will be shown. Otherwise, an icon () will appear to show you that there is an invisible element on your page.

The printenv inspector doesn't give you many options because there isn't anything to specify for printenv. The directive just outputs all known environment variables. To set new values for the environment variables (so you can set up a test environment), click on the "Edit Environment Variables" button.

Since printenv prints out variables one per line, it is suggested you use the directive between <pre> tags for readibility.

For more information about working with environment variables (and XSSI in general), take a look at Electric Types Introduction to XSSI.