eDirectory gives its' users the ability to manage multiple directory site under one license. This allows eDirectory users to house all of their sites in one centralized CMS (Content Management System). Within the sitemanager, you can easily switch between directories you're managing by selecting one of your sites in a drop down box within the interface.
It's important to know that additional sites are NOT additional licenses. Additional licenses are completely different accounts, with different site manager log ins.
If you're still unsure about what situations you would use, here are a few scenerios:
When you should buy additional sites:
When you would buy additional licenses:
In short --- if you want to manage all of your sites under one roof, one interface, you can buy additional sites. If you want to segment them into different site managers or login screens so two people cant access each others' sites through the site manager, you would want to buy additional licenses.