Indexes are primarily a list of published scholarly literature on a topic. They're designed to allow you to systematically look through all of the most important scholarship at once.
This set of resources focuses on resources to help you cover other topics and time periods in more detail. Most are not primarily focused on philosophy, but all will include some material the main indexes won't.
If you search a database and the full article's not there, don't despair. Look for the "FindIt" button: or FindIt@BC. Find It will offer help including: